drivers: adc: add instance versions of the ADC_DT_SPEC* helpers

The instance based version macros were not added when the new
ADC_DT_SPEC* helpers were introduced. This means drivers are forced to
make usage of DT_DRV_INST(inst) macro to obtain a node identifier, the
parameter required by existing macros. Following other driver classes,
provide the ADC_DT_SPEC_INST* family of macros.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>