input: samsung-keypad: Add device tree support

Add device tree based discovery support for Samsung's keypad controller.

Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Donghwa Lee <dh09.lee@samsung.com>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>