pinctrl: samsung: Handle GPIO request and free using pinctrl helpers

This patch adds .request() and .free() operations to gpio_chip of
pinctrl-samsung driver, which call pinctrl request and free helpers to
request and free pinctrl pin along with GPIO pin.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>