firmware: scmi: sandbox test for voltage regulator

Implement sandbox regulator devices for SCMI voltage domains
and test them in DM scmi tests.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
6 files changed