Fix sparse warning:

drivers/pinctrl/aspeed/pinmux-aspeed.c:8:12: warning:
 symbol 'aspeed_pinmux_ips' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20190711142457.37028-1-yuehaibing@huawei.comReviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>