fix:mailbox:bcm-pdc-mailbox:mark symbols static where possible

We get 2 warnings when biuld kernel with W=1:
drivers/mailbox/bcm-pdc-mailbox.c:472:6: warning: no previous prototype for 'pdc_setup_debugfs' [-Wmissing-prototypes]
drivers/mailbox/bcm-pdc-mailbox.c:488:6: warning: no previous prototype for 'pdc_free_debugfs' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>