sandbox: pch: Add a test for the PCH uclass

This uclass currently has no tests. Add a sandbox driver and some simple
tests to provide basic coverage.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: Use "sandbox,pch" for the compatible string, for consistency]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>