thermal: sandbox: Enable thermal uclass for sandbox64 variant

The sandbox64 variant is currently building the sandbox thermal driver
but not the corresponding uclass driver. This results in the sandbox64
variant not booting with the test device tree. Enable building the
thermal uclass for the sandbox64 variant as well.

Also enable the temperature command to allow the test to be run on the
sandbox64 variant.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>