ACPI: CPPC: Make some symbols static

Fix the following sparse warnings:

drivers/acpi/cppc_acpi.c:353:20: warning: symbol 'cppc_mbox_cl' was not declared. Should it be static?
drivers/acpi/cppc_acpi.c:600:5: warning: symbol 'pcc_data_alloc' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>