mfd: Make symbol 'atc260x_i2c_of_match' static

The sparse tool complains as follows:

drivers/mfd/atc260x-i2c.c:45:27: warning:
 symbol 'atc260x_i2c_of_match' was not declared. Should it be static?

This symbol is not used outside of atc260x-i2c.c, so this
commit marks it static.

Fixes: f7cb7fe3

 ("mfd: Add MFD driver for ATC260x PMICs")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>