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>

Move sandbox over to use the reset uclass for reset, instead of a direct
call to do_reset(). This allows us to add tests.

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

Add drivers for sandbox. One can only perform a warm reset (which does
nothing). The other can perform a cold reset or a power reset (the
latter will quit U-Boot). These can be used for testing the reset uclass.

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