gpio: mpc8xxx: Remove *read_reg and *write_reg from struct mpc8xxx_gpio_chip

*read_reg and *write_reg can be removed because at all the places to call
them, we can just use gc->read_reg/gc->write_reg instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>