drivers: retained_mem: Fix using multiple nRF retained memory regions

Fixes an error when using multiple nRF retained memory regions caused by
a missing separating comma.

Signed-off-by: Alex An <aza0337@auburn.edu>

Author: sctanf

drivers/retained_mem/retained_mem_nrf_ram_ctrl.c

@@ -12,7 +12,7 @@
 
 #define _BUILD_MEM_REGION(node_id)		    \
 	{.dt_addr = DT_REG_ADDR(DT_PARENT(node_id)),\
-	 .dt_size = DT_REG_SIZE(DT_PARENT(node_id))}
+	 .dt_size = DT_REG_SIZE(DT_PARENT(node_id))},
 
 struct ret_mem_region {
 	uintptr_t dt_addr;