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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/pcc.c

@@ -568,7 +568,7 @@ static int pcc_mbox_probe(struct platform_device *pdev)
 	return ret;
 }
 
-struct platform_driver pcc_mbox_driver = {
+static struct platform_driver pcc_mbox_driver = {
 	.probe = pcc_mbox_probe,
 	.driver = {
 		.name = "PCCT",