efikamx: update MAINTAINERS for Genesi Efika MX systems

Update maintainer for "efikamx" and "efikasb" to myself.

Signed-off-by: Matt Sealey <matt@genesi-usa.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Marek Vasut <marex@denx.de>