Tested & working:
* Send standard packets
* Receive standard packets (1 FIFO, no filter)
Interoperation between SAM E54 Xplained running this tree and
MicroPython running on STM32F405 Feather with an external
transceiver was also tested.
Many other aspects of a full implementation are not yet present,
such as error detection and recovery.
This introduces the new macro SAM_D5X_E5X. This is mostly the same
as SAMD51 before, except in a few places where a special case for
SAME54 is required
