Input: serio - add support for PS2Mult multiplexer protocol

PS2Mult is a simple serial protocol used for multiplexing several PS/2
streams into one serial data stream. It's used e.g. on TQM85xx series
of boards.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>