gpio: Cleanup DW gpio driver

This patch addresses the following issues:
* Aligns the Kconfig code style with Zephyr projects requirements.
* Removes redundant "depends on" from Kconfig.
* Adds static to the gpio_dw_isr declaration.
* Adds guards to the header files.

Change-Id: I1ae70868f0bda97891cbeb494e5efba1bd537aa1
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>