drivers: adc: Update drivers to use devicetree Kconfig symbol

Update adc drivers to use DT_HAS_<compat>_ENABLED Kconfig symbol
to expose the driver and enable it by default based on devicetree.

We remove 'depend on' Kconfig for symbols that would be implied by
the devicetree node existing.

Signed-off-by: Kumar Gala <galak@kernel.org>