firmware: smccc: Export revision soc_id function

The "SoC ID revision" as provided via the SMCCC SOCID interface can be
valuable information for drivers, when certain functionality depends
on a die revision, for instance.
One example is the sun50i-cpufreq-nvmem driver, which needs this
information to determine the speed bin of the SoC.

Export the arm_smccc_get_soc_id_revision() function so that it can be
called by any driver.

Signed-off-by: Martin Botka <martin.botka@somainline.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>