adc: adc_shell: Rework device name extraction from devicetree

Replace having dts_fixup.h files define DT_ADC_{0..2}_NAME with using
the new devicetree.h macros.  We test to see what driver compat is
enabled via DT_HAS_COMPAT and set DT_DRV_COMPAT to that compat.  Than we
can utilize the DT_INST_LABEL() macro to extract the name of the device.

We also replace the Kconfig ADC_{0..2} symbols with DT_HAS_DRV_INST.
This will allow us to remove those Kconfig symbols as this was the only
usage.

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