retained_mem: ram: use flexible initialization level

Adjust the initialization level depending if a mutex is used or not.

It allows using the drivers pre-kernel for applications that don't use
the mutex anyway.

Sign-off-by: Dawid Niedzwiecki <dawidn@google.com>

Author: niedzwiecki-dawid

drivers/retained_mem/retained_mem_zephyr_ram.c

@@ -126,6 +126,13 @@ static DEVICE_API(retained_mem, zephyr_retained_mem_ram_api) = {
 	.clear = zephyr_retained_mem_ram_clear,
 };
 
+/* Allow to use the driver PRE_KERNEL if mutexes are disabled. */
+#ifdef CONFIG_RETAINED_MEM_MUTEXES
+#define RETAINED_MEM_ZEPHYR_RAM_INIT_LVL POST_KERNEL
+#else
+#define RETAINED_MEM_ZEPHYR_RAM_INIT_LVL PRE_KERNEL_1
+#endif
+
 #define ZEPHYR_RETAINED_MEM_RAM_DEVICE(inst)							\
 	IF_ENABLED(CONFIG_RETAINED_MEM_MUTEXES,							\
 		   (static struct zephyr_retained_mem_ram_data					\
@@ -143,7 +150,7 @@ static DEVICE_API(retained_mem, zephyr_retained_mem_ram_api) = {
 					  (&zephyr_retained_mem_ram_data_##inst), (NULL)	\
 			      ),								\
 			      &zephyr_retained_mem_ram_config_##inst,				\
-			      POST_KERNEL,							\
+			      RETAINED_MEM_ZEPHYR_RAM_INIT_LVL,					\
 			      CONFIG_RETAINED_MEM_INIT_PRIORITY,				\
 			      &zephyr_retained_mem_ram_api);