MAINTAINERS: Add entry for SIOX

Maintenance is split between Gavin who works for Eckelmann and so has
the functional authority, knows the background and history of this bus
system and me who designed most of the actual code with the old
microcontroller code as reference.

Acked-by: Gavin Schenk <g.schenk@eckelmann.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>