boards: arm: nrf-based: move all boards to gpio-as-nreset dt property

Instead of using the about to be deprecated CONFIG_GPIO_AS_PINRESET
option.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
diff --git a/boards/arm/mg100/mg100.dts b/boards/arm/mg100/mg100.dts
index 512ca90..0d33fd3 100644
--- a/boards/arm/mg100/mg100.dts
+++ b/boards/arm/mg100/mg100.dts
@@ -65,6 +65,10 @@
 	status = "okay";
 };
 
+&uicr {
+	gpio-as-nreset;
+};
+
 &gpiote {
 	status = "okay";
 };