mfd: stpmic1: Make stpmic1_regmap_config static

Fix sparse warning:

drivers/mfd/stpmic1.c:62:28: warning:
 symbol 'stpmic1_regmap_config' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>