boards: custom_plank: use DT to enable DCDC

custom_plank is a sample custom board template usable on nRF52840DK.
The DCDC module on the nRF52840 SoC is now enabled using DT, the
Kconfig options are deprecated, so let's keep up to date.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

boards/vendor/custom_plank/Kconfig

@@ -29,6 +29,14 @@
 	};
 };
 
+&reg0 {
+	status = "okay";
+};
+
+&reg1 {
+	regulator-initial-mode = <NRF5X_REG_MODE_DCDC>;
+};
+
 &uicr {
 	gpio-as-nreset;
 };