Input: add Synaptics USB device driver

This patch adds a driver for Synaptics USB touchpad or pointing stick
devices. These USB devices emulate an USB mouse by default, so one can
also use the usbhid driver. However, in combination with special user
space drivers this kernel driver allows one to customize the behaviour
of the device.

An extended version of this driver with support for the cPad background
display can be found at
<http://jan-steinhoff.de/linux/synaptics-usb.html>.

Signed-off-by: Jan Steinhoff <mail@jan-steinhoff.de>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>