Input: tm2-touchkey - add touchkey driver support for TM2

This patch adds support for the TM2 touch key and led functionality.

The driver interfaces with userspace through an input device and reports
KEY_PHONE and KEY_BACK event types. LED brightness can be controlled by
"/sys/class/leds/tm2-touchkey/brightness".

Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Jaechul Lee <jcsing.lee@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>