pinctrl: at91: Add set_multiple GPIO chip feature

This adds the callback for set_multiple.

As this controller has a separate set and clear register, we
can't write directly to PIO_ODSR as this would required a cached
variable and would race with at91_gpio_set.

So build masks for the PIO_SODR and PIO_CODR registers and
write them together.

Signed-off-by: Alexander Stein <alexanders83@web.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>