drivers: adc: add pinctrl driver support

Replace soc-specific pin functions with Zephyr pinctrl api functions for
pin-mux configuration in npcx adc driver. Please notice users need to
configure the corresponding pinctrl nodes in 'pinctrl-0' property in the
adc0 DT node. For example, if ADC0 and ADC2 channels are selected for
the application, please add the follwoings in your board DT layout file.

&adc0 {
	status = "okay";
	/* Use adc0 channel 0 and 2 for 'adc_api' driver tests */
	pinctrl-0 = <&adc0_chan0_gp45
		     &adc0_chan2_gp43>;
	pinctrl-names = "default";
};

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