soc: atmel_sam0: Implement fixup.h for ADC driver

The ADC driver now supports three different implementations. To maintain
readability, this patch implements an adc_fixup.h that permits more
generic access to relevant registers.

This patch also introduces support for a new third shape ADC - as found
in the SAML21 for example.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>