Input: synaptics - add multi-finger and semi-mt support

The Synaptics 2.7 series of touchpads support a mode for reporting two
sets of X/Y/Pressure data (advanced gesture mode). By default, these
devices report only single finger data, depriving userspace of the
nowadays ubiquitous two-finger scroll gesture.

Enabling advanced gesture mode also enables the multi-finger report,
although the device does not claim that capability. Up to three
fingers can be reported this way.

While two or three fingers are touching, the normal packet is
prepended by a reduced finger packet of lower resolution. From the two
packets (which do not represent the actual fingers), the bounding
rectangle of the individual contacts can be extracted.  This
information is sufficient to perform scaling gestures and a limited
form of rotation gesture. The behavior has been coined semi-mt
capability, and is signaled to userspace via the INPUT_PROP_SEMI_MT
device property.

Work to decode the advanced gesture packet: Takashi Iwai.
Cleanup and testing of the original patch: Chase Douglas.
Minor cleanup and testing: Chris Bagwell.
Finalization and semi-mt support: Henrik Rydberg.

Reported-by: Tobyn Bertram
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>