pinctrl: mcp23s08: Implement gpio bulk functions

To speed up some usecases implement reading and writing several IO lines
at once.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230324164957.485924-3-u.kleine-koenig@pengutronix.de


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>