pinctrl: fix dangling comment

This comment was referring to an older PINMUX define, it should
be PINCTRL now.

Reported-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>