Use the fallthrough statement in switch loops

Clang does not seem to work with 'fall through' in comments.

Change-Id: Idcbe373be33ef7247548f856bfaba7ceb7f749b5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/northbridge/intel/sandybridge/raminit_mrc.c

@@ -275,7 +275,7 @@ static void devicetree_fill_pei_data(struct pei_data *pei_data)
 	/* MRC only supports fixed numbers of frequencies */
 	default:
 		printk(BIOS_WARNING, "RAMINIT: Limiting DDR3 clock to 800 Mhz\n");
-		/* fallthrough */
+		__fallthrough;
 	case 400:
 		pei_data->max_ddr3_freq = 800;
 		break;