sandbox: add SCMI power domain protocol support for testing

SCMI power domain management protocol is supported on sandbox
for test purpose. Add fake agent interfaces and associated
power domain devices.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 96cd701..e3a2f9e 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -240,6 +240,7 @@
 CONFIG_PINCTRL_SINGLE=y
 CONFIG_POWER_DOMAIN=y
 CONFIG_SANDBOX_POWER_DOMAIN=y
+CONFIG_SCMI_POWER_DOMAIN=y
 CONFIG_DM_PMIC=y
 CONFIG_PMIC_ACT8846=y
 CONFIG_DM_PMIC_PFUZE100=y