pinctrl: fsl: Make pinctrl_ipc_handle static

Fix sparse warning:

drivers/pinctrl/freescale/pinctrl-scu.c:38:19: warning:
 symbol 'pinctrl_ipc_handle' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>