HID: rmi: Support touchpads with external buttons

The external buttons on HID touchpads are connected as pass through devices and
button events are not reported in the rmi registers. As a result on these
devices we need to allow the HID generic desktop button events to be processed
by hid-input. Unfortunately, there is no way to query the touchpad to determine
that it has pass through buttons so the RMI_DEVICE_HAS_PHYS_BUTTONS should be
set manually when adding the device to rmi_id[].

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>