gpio: pcf857x: share 8/16 bit access functions

This patch adds 8/16 bit write/read functions,
and shared gpio input/output/set/get functions.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>