cpufreq: scmi: Skip SCMI devices that aren't used by the CPUs

Currently, all SCMI devices with performance domains attempt to register
a cpufreq driver, even if their performance domains aren't used to
control the CPUs. The cpufreq framework only supports registering a
single driver, so only the first device will succeed. And if that device
isn't used for the CPUs, then cpufreq will scale the wrong domains.

To avoid this, return early from scmi_cpufreq_probe() if the probing
SCMI device isn't referenced by the CPU device phandles.

This keeps the existing assumption that all CPUs are controlled by a
single SCMI device.

Signed-off-by: Mike Tipton <quic_mdtipton@quicinc.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>