firmware: smci: sandbox test for SCMI reset controllers

Add tests for SCMI reset controllers. A test device driver
sandbox-scmi_devices.c is used to get reset resources, allowing further
resets manipulation.

Change sandbox-smci_agent to emulate 1 reset controller exposed through
an agent. Add DM test scmi_resets to test this reset controller.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>