firmware: scmi: sandbox test for SCMI clocks

firmware: scmi: sandbox test for SCMI clocks

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

Change sandbox-smci_agent to emulate 3 clocks exposed through 2 agents.
Add DM test scmi_clocks to test these 3 clocks.
Update DM test sandbox_scmi_agent with load/remove test sequences
factorized by {load|remove}_sandbox_scmi_test_devices() helper functions.

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>