[nrf fromtree] samples: nordic: system_off: add retained_mem variant for nRF5340 DK

nRF5340 SoC supports retained_mem driver, which can be
utilized for RAM retention in System OFF sleep mode.
Add sample configuration confirming this.

Signed-off-by: Nikodem Kastelik <[email protected]>
(cherry picked from commit 50f6142)

Author: nika-nordic

samples/boards/nordic/system_off/boards/nrf5340dk_nrf5340_cpuapp.overlay

@@ -0,0 +1,22 @@
+/ {
+	sram0_image@2006f000 {
+		compatible = "zephyr,memory-region", "mmio-sram";
+		reg = <0x2006f000 DT_SIZE_K(4)>;
+		zephyr,memory-region = "RetainedMem";
+		status = "okay";
+
+		retainedmem0: retainedmem {
+			compatible = "zephyr,retained-ram";
+			status = "okay";
+		};
+	};
+
+	aliases {
+		retainedmemdevice = &retainedmem0;
+	};
+};
+
+&sram0_image {
+	/* Shrink SRAM size to avoid overlap with retained memory region */
+	reg = <0x20000000 DT_SIZE_K(444)>;
+};

samples/boards/nordic/system_off/sample.yaml

@@ -29,6 +29,7 @@ tests:
     platform_allow:
       - nrf52840dk/nrf52840
       - nrf52dk/nrf52832
+      - nrf5340dk/nrf5340/cpuapp
       - nrf54l15dk/nrf54l05/cpuapp
       - nrf54l15dk/nrf54l10/cpuapp
       - nrf54l15dk/nrf54l15/cpuapp