pinctrl/gpio: Unify namespace for cross-calls

The pinctrl_request_gpio() and pinctrl_free_gpio() break the nice
namespacing in the other cross-calls like pinctrl_gpio_foo().
Just rename them and all references so we have one namespace
with all cross-calls under pinctrl_gpio_*().

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