gpio: samsung: Drop support for Exynos SoCs

GPIO support on Exynos SoCs is provided by pinctrl-samsung driver,
leaving all the support code in gpio-samsung driver unused. This dead
code can be safely removed and so it is done by this patch.
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>