sandbox: Add asm/cache.h for sandbox

This file is required by the new DMA buffer alignment macro.

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>