soc: atmel_sam0: Add SAMD51

This adds supoprt for the Atmel SAMD51 SoC.

The SAME5x/SAMD5x is a line of Cortex-M4F MCUs that share peripherals
with the sam0 Cortex-M0+ and saml1x Cortex-M23 parts.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>