pinctrl: pistachio: Make it as an option

So it will be avilable for generic MIPS kernel.

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Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
v3: Depend on OF as well
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>