firmware: scmi: fix sandbox and related tests for clock discovery

Updates sandbox SCMI clock driver and tests since enabling CCF will
mandate clock discovery that is all exposed SCMI clocks shall be
discovered at initialization. For this reason, sandbox SCMI clock
driver must emulate all clocks exposed by SCMI server, not only those
effectively consumed by some other U-Boot devices.

Therefore the sandbox SCMI test driver exposes 3 clocks (IDs 0, 1 and 2)
and sandbox SCMI clock consumer driver gets 2 of them.

Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>