HID: Add driver for USB ELAN Touchpad

This is driver for usb touchpad found on HP Pavilion x2 10-p0xx laptop. On this
device keyboard and touchpad connected as a single usb device with two
interfaces: keyboard, which exposes ordinary keys and second interface is
touchpad which also contains FlightMode button and audio mute led (which
physically placed on keyboard for some reason).

Initially, this touchpad works in mouse emulation mode, this driver will switch
it to touchpad mode, which can track 5 fingers and can report coordinates for
two of them.

Signed-off-by: Alexandrov Stansilav <neko@nya.ai>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>