boards: nrf52*: use DT to configure regulators

Migrate from the deprecated Kconfig option to configure regulators
of the nRF52 SoC series to DTS description.

This change aligns the sdk-nrf repository with the upstream changes
that were introduced in the following commit:

zephyrproject-rtos/zephyr@9097d5dac5a354c4fc0

Ref: NCSDK-29757

Signed-off-by: Kamil Piszczek <[email protected]>
(cherry picked from commit fc95425)

Author: kapi-no

boards/nordic/nrf52820dongle/Kconfig

@@ -63,6 +63,10 @@
 	};
 };
 
+&reg1 {
+	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
+};
+
 &uicr {
 	gpio-as-nreset;
 };

boards/nordic/nrf52820dongle/nrf52820dongle_nrf52820.dts

@@ -63,6 +63,10 @@
 	};
 };
 
+&reg1 {
+	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
+};
+
 &uicr {
 	nfct-pins-as-gpios;
 	gpio-as-nreset;

boards/nordic/nrf52833dongle/Kconfig

@@ -37,6 +37,10 @@
 	status = "okay";
 };
 
+&reg1 {
+	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
+};
+
 &adc {
 	status = "okay";
 };

boards/nordic/nrf52833dongle/nrf52833dongle_nrf52833.dts

@@ -25,6 +25,10 @@
 	};
 };
 
+&reg {
+	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
+};
+
 &adc {
 	status = "okay";
 };

boards/nordic/nrf52840gmouse/Kconfig

@@ -25,6 +25,10 @@
 	};
 };
 
+&reg {
+	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
+};
+
 &adc {
 	status = "okay";
 };