[nrf fromlist] drivers: retained_mem: nrf: make ram_ctrl variant explicit

Updated retained_mem driver variant exposes vendor-specific API,
rather than executing implicitly during boot.

Upstream PR #: 82262

Signed-off-by: Nikodem Kastelik <[email protected]>
(cherry picked from commit 0819c8e)

Author: nika-nordic

drivers/retained_mem/retained_mem_nrf_ram_ctrl.c

@@ -6,6 +6,7 @@
 
 #include <zephyr/devicetree.h>
 #include <zephyr/kernel.h>
+#include <zephyr/drivers/retained_mem/nrf_retained_mem.h>
 
 #include <helpers/nrfx_ram_ctrl.h>
 
@@ -22,7 +23,7 @@ static const struct ret_mem_region ret_mem_regions[] = {
 	DT_FOREACH_STATUS_OKAY(zephyr_retained_ram, _BUILD_MEM_REGION)
 };
 
-static int retained_mem_nrf_init(void)
+int z_nrf_retained_mem_retention_apply(void)
 {
 	const struct ret_mem_region *rmr;
 
@@ -33,5 +34,3 @@ static int retained_mem_nrf_init(void)
 
 	return 0;
 }
-
-SYS_INIT(retained_mem_nrf_init, PRE_KERNEL_1, 0);

include/zephyr/drivers/retained_mem/nrf_retained_mem.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2024 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_DRIVERS_RETAINED_MEM_NRF_RETAINED_MEM_H
+#define ZEPHYR_INCLUDE_DRIVERS_RETAINED_MEM_NRF_RETAINED_MEM_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <zephyr/drivers/retained_mem.h>
+
+#if defined(CONFIG_RETAINED_MEM_NRF_RAM_CTRL) || defined(__DOXYGEN__)
+/** @brief Apply memory retention settings.
+ *
+ * Memory retention settings to apply are derived from devicetree configuration.
+ *
+ * @retval 0 if the retention settings were applied successfully.
+ * @retval -ENOTSUP if retention configuration is not present in devicetree.
+ */
+int z_nrf_retained_mem_retention_apply(void);
+#else
+static inline int z_nrf_retained_mem_retention_apply(void)
+{
+	return -ENOTSUP;
+}
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_INCLUDE_DRIVERS_RETAINED_MEM_NRF_RETAINED_MEM_H */