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>

dts/bindings/adc/nuvoton,npcx-adc.yaml

@@ -16,6 +16,10 @@ properties:
     required: true
   pinctrl-names:
     required: true
+  vref-mv:
+    type: int
+    required: true
+    description: ADC reference voltage (Unit:mV)
   channel-count:
     type: int
     required: true