Input: add IMS Passenger Control Unit driver

The PCU is a device installed in the armrest of a plane seat and
is connected to IMS Rave Entertainment System. It has a set of control
buttons (Volume Up/Down, Attendant, Lights, etc) on one side and
gamepad-like controls on the other side.

Originally the device was handled from userspace and because of that
it presents itself on USB bus as a CDC-ACM modem device that however
can not make calls. However the custom handling is not as convenient
as using standard input subsystem facilities. If it was pure input
device it would be possible to continue using userspace solution
(moving it over to uinput), but the device also has backlighted keys
which can not be supported via uinput.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>