gpio: pcf857x: Implement get_multiple/set_multiple methods

This change allows the GPIO core to read/change multiple pins in a
single driver call and subsequent I2C transfer. It helps a lot with
PCF857x devices, since their I2C protocol always reads/changes all
existing pins anyway. Therefore, when the GPIO client code does a bulk
operation on multiple pins, the driver makes a single I2C transfer.

Signed-off-by: Radu Rendec <radu.rendec@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>