[nrf fromtree] tests: drivers: retained_mem: Run test on nrf54l20pdk

Add overlays to execute retained_mem test on nrf54l20pdk.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>
(cherry picked from commit eaf9992)

Author: nordic-segl

tests/drivers/retained_mem/api/boards/nrf54l20pdk_nrf54l20_cpuapp.conf

@@ -0,0 +1 @@
+CONFIG_POWEROFF=y

tests/drivers/retained_mem/api/boards/nrf54l20pdk_nrf54l20_cpuapp.overlay

@@ -0,0 +1,25 @@
+/ {
+	cpuapp_sram@2007ec00 {
+		compatible = "zephyr,memory-region", "mmio-sram";
+		reg = <0x2007ec00 DT_SIZE_K(4)>;
+		zephyr,memory-region = "RetainedMem";
+		status = "okay";
+
+		retainedmem0: retainedmem {
+			compatible = "zephyr,retained-ram";
+			status = "okay";
+		};
+	};
+
+	aliases {
+		retainedmemtestdevice = &retainedmem0;
+	};
+};
+
+&cpuapp_sram {
+	/* Shrink SRAM size to avoid overlap with retained memory region:
+	 * 511 - 4 = 507KB = 0x7ec00
+	 */
+	reg = <0x20000000 DT_SIZE_K(507)>;
+	ranges = <0x0 0x20000000 0x7ec00>;
+};

tests/drivers/retained_mem/api/testcase.yaml

@@ -12,6 +12,7 @@ tests:
       - nrf54l15dk/nrf54l05/cpuapp
       - nrf54l15dk/nrf54l10/cpuapp
       - nrf54l15dk/nrf54l15/cpuapp
+      - nrf54l20pdk/nrf54l20/cpuapp
     tags:
       - drivers
       - retained_mem