soc: atmel_sam0: The ADC reference is locked while enabled for SAML21

For SAML21-based parts, the REFCTRL register is locked while the ADC is
enabled. Permit some parts to declare that they need the ADC to be
disabled before modifying REFCTRL.

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