ARM: dts: stm32: add ADC pins used for stm32mp157c-ed1

Define adc1_in6 pin used on stm32mp157c eval board.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp157-pinctrl.dtsi

@@ -137,6 +137,12 @@ gpiok: gpio@5000c000 {
 				status = "disabled";
 			};
 
+			adc1_in6_pins_a: adc1-in6 {
+				pins {
+					pinmux = <STM32_PINMUX('F', 12, ANALOG)>;
+				};
+			};
+
 			adc12_ain_pins_a: adc12-ain-0 {
 				pins {
 					pinmux = <STM32_PINMUX('C', 3, ANALOG)>, /* ADC1 in13 */