Schwabe/silence register warnings clang (#3940)

* Ignore deprecated register warning for the the STM include files

Ignore the warning with the use of #pragma which is not elegant but otherwise we would have to change the vendor files

* Remove usage of register in C/C++ files.

Modern compilers ignore it anyway and C++11 actually deprecates it. Removing also removes a number of clang warnings about this

radio/src/targets/sky9x/i2c_driver.cpp

@@ -51,7 +51,7 @@ bool i2cInit()
   //Enable Peripheral Clock
   PMC->PMC_PCER0 |= 0x00080000L;
   //Enable TWI PIOs
-  register Pio *pioptr;
+  Pio *pioptr;
   pioptr = PIOA;
   pioptr->PIO_ABCDSR[0] &= ~0x00000018;        // Peripheral A
   pioptr->PIO_ABCDSR[1] &= ~0x00000018;        // Peripheral A