gpio: pl061: Add pl061_gpio_free()

commit 39b70ee051 "gpio: pl061: bind pinctrl by gpio request"
adds pl061_gpio_request() to request pinctrl.
Thus we need to add corresponding pl061_gpio_free() to free the pin.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>