gk7205v200: add an OTG profile that makes the camera a USB webcam (#113)

Adds gk7205v200_otg_generic, a firmware profile that puts the single DWC3
controller into dual-role so the board can either consume a USB webcam
(existing usbcam path, host role) or present itself to a PC as one (UVC
gadget, device role). The roles are mutually exclusive -- xHCI HCSPARAMS1
reports MaxPorts = 1 -- but usb-mode switches between them at runtime, so
one image covers both and no reboot is needed.

Kernel side, three patches:

0015 sets dr_mode = "otg" in the DTS so dwc3 registers both an xHCI host
and a UDC.

0016 makes device mode actually work on a part that has only ever run as
a host. Three fixes: re-run dwc3_event_buffers_setup() on gadget start
(an xHCI reset wipes GEVNTADR), soft-reset the core before taking the
lock so the PHY is re-initialised (without it high-speed chirp fails),
and swallow the run_stop timeout on pullup(0), which otherwise
permanently poisons cdev->deactivations.

0017 disables the vendor's UVC_SG_REQ path. It hands a whole frame to the
UDC as one scatter-gather request; dwc3 chains those sg entries into a
single isochronous service interval, which on a full-speed link is one
1023-byte packet, so every frame larger than that reached the host
truncated and H.264 decoders reported "bytestream -5" on roughly half the
frames. Goke never shipped this path enabled -- CONFIG_USB_CONFIGFS_F_UVC
is unset in every vendor defconfig, including gk7605v100's, and the SDK
carries no userspace UVC sample -- so it was never exercised. The
mainline path (32 plain one-packet requests) is used instead.

Userspace side: usb-mode switches roles with the unbind/flip/bind
sequencing dwc3_set_mode() does not do on its own, uvc-gadget-setup
composes the configfs gadget, and S72usbmode forces a concrete role at
boot -- not optional, since dr_mode = "otg" leaves GCTL.PRTCAPDIR in a
state this 4.9 dwc3 never moves off by itself.

The descriptors advertise exactly the one format, frame size and interval
majestic is configured to serve, read from /etc/majestic.yaml. majestic
answers every PROBE/COMMIT with its own bFormatIndex/bFrameIndex because
it has one stream and cannot follow the host, so anything extra is an
invitation to select something that will not arrive: asking for MJPEG got
raw H.264 (an SPS where a JPEG SOI belongs, garbage to every consumer),
and asking for 1080p got 720p.

Profile drops the air-unit packages (wifibroadcast, rtl8812au, mavfwd,
mavlink-router, vtund, linux-firmware, wireless-tools) and the whole
wireless kernel stack, which pays for usb_f_uvc.ko and for keeping the
full sensor list -- the fpv trim to IMX307/IMX335 is keyed on
OPENIPC_VARIANT="fpv", and a general-purpose USB webcam should not be
sensor-locked.

Measured on a gk7205v200 lab board over a full-speed link: H.264
1280x720 at 30 fps with zero decoder errors and no dropped buffers, and
MJPEG delivering valid JPEGs at the jpeg channel's own rate.

Requires a majestic built with ENABLE_UVC_GADGET to feed the gadget;
that is a separate change.

.github/workflows/master.yml

@@ -176,6 +176,9 @@ jobs:
           - hi3516cv300_mini
           - hi3518ev200_mini
 
+          # OTG (camera presents itself to a PC as a USB webcam)
+          - gk7205v200_otg_generic
+
     steps:
       - name: Checkout source
         uses: actions/checkout@v4