pinctrl: pistachio: Make it as an option

So it will be avilable for generic MIPS kernel.

--

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
v3: Depend on OF as well
Link: https://lore.kernel.org/r/20210721030134.10562-7-jiaxun.yang@flygoat.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>