refactor(sds): modify log level for region validity

Modify the log level from WARNING to VERBOSE for the SDS region
validity check. An invalid region causes the initialization step
to fail, but normally it's only a temporary condition as the
actual initialization of the region (such as adding a valid region
descriptor structure) can happen asynchronously in another system
component. The goal of this tiny modification is to avoid flooding
the log with this message when we're waiting in a loop for the
region initialization to happen.

Change-Id: I180e35e25df3f31bbc816e6421ded17ba6ae1d85
Signed-off-by: David Vincze <david.vincze@arm.com>

drivers/arm/css/sds/sds.c

@@ -250,7 +250,7 @@ int sds_init(unsigned int region_id)
 	uintptr_t sds_mem_base = sds_regions[region_id].base;
 
 	if (!IS_SDS_REGION_VALID(sds_mem_base)) {
-		WARN("SDS: No valid SDS Memory Region found\n");
+		VERBOSE("SDS: No valid SDS Memory Region found\n");
 		return SDS_ERR_FAIL;
 	}