drivers: adc: add adc support in npcx7 series

NPCX7 includes a 10-bit resolution Analog-to-Digital Converter (ADC). Up
to 10 voltage inputs can be measured and a internal voltage reference
(VREF), 2.816V (typical) is used for measurement. It can be triggered
automatically in Autoscan mode. Each input channel is assigned a
separate result register, which is updated at the end of the conversion.

The CL also includes:
— Add npcx adc device tree declarations.
— Zephyr adc api implementation.
— Add adc definitions of npcx7 in
  tests/drivers/adc/adc_api/src/test_adc.c for supporting test suites.

Signed-off-by: Mulin Chao <MLChao@nuvoton.com>