gpio: 74x164: Implement gpiochip.set_multiple()

This allows to set multiple outputs using a single SPI transfer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>