PCC: Initialize PCC Mailbox earlier at boot

This change initializes the PCC Mailbox earlier than
the ACPI processor driver. This enables drivers introduced
in follow up patches (e.g. CPPC) to be probed via the ACPI
processor driver interface. The CPPC probe requires the PCC
channel to be initialized for it to query each CPUs performance
capabilities.

Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Reviewed-by: Al Stone <al.stone@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>