Input: gpio_keys - switch to using generic device properties

Make use of the device property API in this driver so that both OF based
systems and ACPI based systems can use this driver.

Suggested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Suggested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>