MAINTAINERS: add maintainer entry for TDA998x driver

Add a maintainers entry for the TDA998x driver.  Rob Clark has handed
this driver over to me to look after.

Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>