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>