samples: drivers: adc: Add rpi_pico support

/*

 * SPDX-License-Identifier: Apache-2.0

 *

 * Copyright (c) 2022 TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>

 */

#include <zephyr/dt-bindings/adc/adc.h>

/ {

	zephyr,user {

		io-channels = <&adc 0>;

	};

};

&adc {

	#address-cells = <1>;

	#size-cells = <0>;

	channel@0 {

		reg = <0>;

		zephyr,gain = "ADC_GAIN_1";

		zephyr,reference = "ADC_REF_INTERNAL";

		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;

		zephyr,resolution = <12>;

	};

};