gpio: pl061: bind pinctrl by gpio request

Add the pl061_gpio_request() to request pinctrl. Create the logic
between pl061 gpio driver and pinctrl (pinctrl-single) driver.

While a gpio pin is requested, it will request pinctrl driver to
set that pin with gpio function mode. So pinctrl driver should
append .gpio_request_enable() in pinmux_ops.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>