drivers: adc: sam0: Fix adc_reference implementation

The current sam0 adc driver not implement correctly the adc_reference
enum values. This try homonize adc input referece by tracking VDDANA
at ADC_REF_VDD_1. The ADC_REF_VDD_1_2 were fixed with correct INTVCCx
channel selection.

Fixes #45443

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>