pinctrl: at91-pio4: implement .get_multiple and .set_multiple

Implement .get_multiple and .set_multiple to allow reading or setting
multiple pins simultaneously. Pins in the same bank will all be switched at
the same time, improving synchronization and performances.

Keep the driver future proof by allowing its use on 64bits platforms if
they ever appear with this IP and we end up with a mismatch between
ATMEL_PIO_NPINS_PER_BANK and BITS_PER_LONG.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20190918113657.25998-1-alexandre.belloni@bootlin.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>