sandbox: Correct operaion of 'reset' command

Currently 'reset' only works with the test device tree. When run without a
device tree, or with the normal device tree, the following error is
displayed:

   Reset not supported on this platform

Fix the driver and the standard device tree to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>