drivers: adc: adc_sam0: rework devicetree support

Rework the devicetree to utilize new DT_INST macros and extract per
instance data for clocks from devicetree.

We add a property ('calib-offset') for the SAM{D,E}5x family of SoCs
that is the bit position offset from ADC0 BIASCOMP in the NVM Software
Calibration Area Mapping.  For ADC0 this is typically 0 and for ADC1
this will be 14.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>