arm-cci: Rearrange code for splitting PMU vs driver code

No functional changes, only code re-arrangements for easier split of the
PMU code vs low level driver code. Extracts the port handling code
to cci_probe_ports().

Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>