fix(core): support magic bootloader values for nRF52833 (#3163)

Enable ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_ADAFRUIT_NRF52 for
nRF52833, add nrf52833_uf2_boot_mode.dtsi as a copy of the nRF52840 file
for easy inclusion.

Enable retention for all nrfmicro subvariants (as this is the only
in-tree board that uses the nRF52833).

This fixes &bootloader for nRF52833 boards post-4.1 upgrade.

app/dts/common/nordic/nrf52833_uf2_boot_mode.dtsi

@@ -0,0 +1,29 @@
+
+&gpregret1 {
+    adafruit_boot_retention: retention@0 {
+        compatible = "zephyr,retention";
+        status = "okay";
+        reg = <0x0 0x1>;
+    };
+};
+
+/ {
+    chosen {
+        zephyr,boot-mode = &boot_retention;
+        zmk,magic-boot-mode = &adafruit_boot_retention;
+    };
+
+    magic_mapper {
+        compatible = "zmk,bootmode-to-magic-mapper";
+        status = "okay";
+
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        boot_retention: retention@0 {
+            compatible = "zephyr,retention";
+            status = "okay";
+            reg = <0x0 0x1>;
+        };
+    };
+};