sandbox: Change md command to use map_physmem

Sandbox wants to support commands which use memory. The map_physmen()
call provides this feature, so should be used more consistently in
U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Tested-by: Matthias Weisser <weisserm@arcor.de>