gpio: samsung: Skip initialization if device tree is present

Since this driver does not handle GPIO on device tree enabled platforms
any more, it should be bypassed whenever device tree is available, to
not conflict with the new pinctrl-samsung driver.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>