mailbox: pcc: make pcc_mbox_driver static

Fix the following sparse warning:

drivers/mailbox/pcc.c:571:24: warning: symbol 'pcc_mbox_driver' was not
declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>