CHROMIUM: Input: atmel_mxt_ts - add device id for touchpad variant

CHROMIUM: Input: atmel_mxt_ts - add device id for touchpad variant

This same driver can be used by atmel based touchscreens and touchpads
(buttonpads) by instantiating the i2c device as a "atmel_mxt_tp".

This will cause the driver to perform some touchpad specific
initializations, such as:
  * register input device name "Atmel maXTouch Touchpad" instead of
  Touchscreen.
  * register BTN_LEFT & BTN_TOOL_* event types.
  * register axis resolution (as a fixed constant, for now)
  * register BUTTONPAD property
  * process GPIO buttons using reportid T19

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:27714
TEST=builds clean; atmel tp works using ddc lines and cmt

Change-Id: Ifb5cff5667156ca48c1ca90e5e8eea0c434480df
Reviewed-on: https://gerrit.chromium.org/gerrit/17959
Commit-Ready: Benson Leung <bleung@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Tested-by: Benson Leung <bleung@chromium.org>