soc: atmel: add base support for C2x SOC

Adds Atmel SAMC20 and SAMC21 soc. C series is based on Cortex-M0+.
C21 contains CAN interface.

The init routines are same for SAMC20 and SAMC21. They use one
clock OSC48M without configuration.

The code is inspirated from atmel_sam0/samd21.

Signed-off-by: Kamil Serwus <kserwus@gmail.com>