spl: mmc: properly annotate fallthrough

Depending on the various MMC boot configurations, we might end up with
trying filesystem mode when a raw image boot failed. This fall-through
in the switch/case statement is explained in a comment, but this is not
visible to the compiler, which still will complain.

Add the proper compiler-visible annotation, to allow enabling the
compiler check in the future.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

common/spl/spl_mmc.c

@@ -411,6 +411,7 @@ int spl_mmc_load(struct spl_image_info *spl_image,
 			return 0;
 #endif
 		/* If RAW mode fails, try FS mode. */
+		fallthrough;
 #ifdef CONFIG_SYS_MMCSD_FS_BOOT
 	case MMCSD_MODE_FS:
 		debug("spl: mmc boot mode: fs\n");