pinctrl/coh901: use clk_prepare_[en|dis]able()

The COH901 GPIO driver did not prepare or unprepare its
clock, so let us fix it to do so.

Acked-by: Pankaj Jangra <jangra.pankaj9@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>