driver: adc: npcx: add multi-device support in npcx adc driver

Add multi-device support in npcx adc driver since there is more than one
adc module in npcx4 series. And each adc's reference voltage might be
different, this CL introduces the `vref-mv` prop. to select its own
reference voltage.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>
Signed-off-by: Kate Yen <htyen@nuvoton.com>