sandbox: implement invalidate_icache_all()

sandbox: implement invalidate_icache_all()

Before executing code that we have loaded from a file we need to flush the
data cache and invalidate the instruction flash.

Implement functions flush_cache() and invalidate_icache_all().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>