Update TDA998x maintainer entry

Update my TDA998x HDMI encoder MAINTAINERS entry to include the
dt-bindings header, and a keyword pattern to catch patches containing
the DT compatible.  Also change the status to "maintained" rather than
"supported".

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>