x86: sandbox: pch: Add a CONFIG option for PCH

At present this uclass is selected only on x86. In order to add a test for
it, it must also support sandbox. Create a new CONFIG_PCH option and
enable it on x86 and sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>