ARM: riscpc: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: rpc_defconfig arm):

arch/arm/mach-rpc/riscpc.c: In function ‘parse_tag_acorn’:
arch/arm/mach-rpc/riscpc.c:48:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   vram_size += PAGE_SIZE * 256;
   ~~~~~~~~~~^~~~~~~~~~~~~~~~~~
arch/arm/mach-rpc/riscpc.c:49:2: note: here
  case 256:
  ^~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

arch/arm/mach-rpc/riscpc.c

@@ -46,6 +46,7 @@ static int __init parse_tag_acorn(const struct tag *tag)
 	switch (tag->u.acorn.vram_pages) {
 	case 512:
 		vram_size += PAGE_SIZE * 256;
+		/* Fall through - ??? */
 	case 256:
 		vram_size += PAGE_SIZE * 256;
 	default: