sandbox: add ADC driver

This commit adds implementation of Sandbox ADC device emulation.
The device provides:
- single and multi-channel conversion
- 4 channels with predefined conversion output data
- 16-bit resolution

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

arch/sandbox/dts/sandbox_pmic.dtsi

@@ -55,7 +55,7 @@
 		regulator-always-on;
 	};
 
-	buck2 {
+	buck2: buck2 {
 		regulator-name = "SUPPLY_3.3V";
 		regulator-min-microvolt = <3300000>;
 		regulator-max-microvolt = <3300000>;

arch/sandbox/dts/test.dts

@@ -189,6 +189,12 @@
 		};
 	};
 
+	adc@0 {
+		compatible = "sandbox,adc";
+		vdd-supply = <&buck2>;
+		vss-microvolts = <0>;
+	};
+
 	leds {
 		compatible = "gpio-leds";