HID: logitech-hidpp: support combo keyboard touchpad TK820

The TK820 presents both a keyboard and a touchpad on the same
physical (and logical device). Use the generic hid-input
processing for the keyboard part. The keyboard input device is created
when the receiver is plugged in, so no events are missed on connect.

When the device actaully connects, we can set it to use the raw
multitouch reporting to have a consistent user experience accross
all Logitech touchpads.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>