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>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 5da8d16..c9318d4 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -219,6 +219,7 @@
 CONFIG_REGULATOR_RK8XX=y
 CONFIG_REGULATOR_S5M8767=y
 CONFIG_DM_REGULATOR_SANDBOX=y
+CONFIG_DM_REGULATOR_SCMI=y
 CONFIG_REGULATOR_TPS65090=y
 CONFIG_DM_PWM=y
 CONFIG_PWM_SANDBOX=y