Input: add MT_TOOL_PALM

Currently there are only two "tools" that can be specified by a multi-touch
driver: MT_TOOL_FINGER and MT_TOOL_PEN. In working with Elan (The touch
vendor) and discussing their next-gen devices it seems that it will be
useful to have more tools so that their devices can give the upper layers
of the stack hints as to what is touching the sensor.

In particular they have new experimental firmware that can better
differentiate between palms vs fingertips and would like to plumb a patch
so that we can use their hints in higher-level gesture soft- ware.  The
firmware on the device can reasonably do a better job of palm detection
because it has access to all of the raw sensor readings as opposed to just
the width/pressure/etc that are exposed by the driver.  As such, the
firmware can characterize what a palm looks like in much finer-grained
detail and this change would allow such a device to share its findings with
the kernel.

Signed-off-by: Charlie Mooney <charliemooney@chromium.org>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>