sandbox: Add a dummy dcache_status() function

This adds dcache_status() so that code using it can build
without error on sandbox. This is required in preparation
of adding cache handling into get_ram_size function.

Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>