ACPI / gpio: make acpi_gpiochip_parse_own_gpio static

Fixes the following sparse warning:

drivers/gpio/gpiolib-acpi.c:863:18: warning:
 symbol 'acpi_gpiochip_parse_own_gpio' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>