pinctrl: mvebu: update use "nand" function for "rb" pin

The Armada 98dx3236 SoCs don't have a different MPP sel value for nand
specific pins so "dev" was technically correct. But all the other Armada
SoCs use "nand" in their dts and the pin is specific to the nand
interface so use "nand" for the function name.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>