mb/google/poppy/variants/nami: Add fallthrough comment

This fallthrough is intentional (see commit 2257a35862 - Perform PL2
setting for syndra), so add a comment to make that explicit.

Change-Id: I57fe1e08f59aed12544cd2a71f1e0464f432f03b
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1397063
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33156
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/poppy/variants/nami/mainboard.c

@@ -252,6 +252,7 @@ void variant_devtree_update(void)
 	case SKU_6_SYNDRA:
 	case SKU_7_SYNDRA:
 		pl2_id = PL2_ID_SONA_SYNDRA;
+		/* fallthrough */
 	case SKU_0_VAYNE:
 	case SKU_1_VAYNE:
 	case SKU_2_VAYNE: