efi_loader: Use map_sysmem() in bootefi command

The bootefi command gets a few addresses as values passed in. In sandbox,
these values are in U-Boot address space, so we need to make sure we
explicitly call map_sysmem() on them to be able to access them.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>