gpio/samsung: Add device tree support for EXYNOS4

As gpio chips get registered, a device tree node which represents the
gpio chip is searched and attached to it. A translate function is also
provided to convert the gpio specifier into actual platform settings
for pin function selection, pull up/down and driver strength settings.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
[kgene.kim@samsung.com: fixed build error]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>