Input: synaptics-rmi4 - add sysfs interfaces for hardware IDs

These attributes provide various bits of information which may be enumerated
under the RMI4 protocol to user space.

This may be useful for displaying the particular version which is in use, or
selecting the correct firmware to flash.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>