[nrf fromlist] samples: drivers: adc: add support for internal nRF54L SAADC inputs

SAADC peripheral for nRF54L Series allows to choose internal
voltages as positive inputs and samples shall showcase that.

Upstream PR: zephyrproject-rtos/zephyr#79330

Signed-off-by: Nikodem Kastelik <[email protected]>
(cherry picked from commit c67b5fa3217b21bfe1604126f06cceb2a1f7c3a9)

Author: nika-nordic

samples/drivers/adc/adc_dt/boards/nrf54l15pdk_nrf54l15_cpuapp.overlay

@@ -6,7 +6,7 @@
 
 / {
 	zephyr,user {
-		io-channels = <&adc 0>, <&adc 1>, <&adc 7>;
+		io-channels = <&adc 0>, <&adc 1>, <&adc 2>, <&adc 7>;
 	};
 };
 
@@ -33,6 +33,16 @@
 		zephyr,oversampling = <8>;
 	};
 
+	channel@2 {
+		reg = <2>;
+		zephyr,gain = "ADC_GAIN_1";
+		zephyr,reference = "ADC_REF_INTERNAL";
+		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
+		zephyr,input-positive = <NRF_SAADC_VDD>;
+		zephyr,resolution = <12>;
+		zephyr,oversampling = <8>;
+	};
+
 	channel@7 {
 		reg = <7>;
 		zephyr,gain = "ADC_GAIN_1";

samples/drivers/adc/adc_sequence/boards/nrf54l15pdk_nrf54l15_cpuapp.overlay

@@ -6,7 +6,7 @@
 
 / {
 	zephyr,user {
-		io-channels = <&adc 0>, <&adc 1>, <&adc 7>;
+		io-channels = <&adc 0>, <&adc 1>, <&adc 2>, <&adc 7>;
 	};
 };
 
@@ -39,6 +39,16 @@
 		zephyr,oversampling = <8>;
 	};
 
+	channel@2 {
+		reg = <2>;
+		zephyr,gain = "ADC_GAIN_1";
+		zephyr,reference = "ADC_REF_INTERNAL";
+		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
+		zephyr,input-positive = <NRF_SAADC_DVDD>; /* 0.9 V internal */
+		zephyr,resolution = <12>;
+		zephyr,oversampling = <8>;
+	};
+
 	channel@7 {
 		reg = <7>;
 		zephyr,gain = "ADC_GAIN_1";