sandbox: Unmap old tags

sandbox: Unmap old tags

So far unmapping has not been implemented. This means that if one test
maps a pointer to an address with map_sysmem(), then a second test can
use that same pointer, by mapping the address back to a pointer with
map_to_sysmem(). This is not really desirable, even if it doesn't
cause any problems at the moment.

Implement unmapping, to clean this up.

Signed-off-by: Simon Glass <sjg@chromium.org>