media: MAINTAINERS: refurbish MEDIATEK JPEG DRIVER section

Commit a16ce2f3 ("media: dt-bindings: mediatek: convert mtk jpeg
decoder/encoder to yaml") converts mediatek-jpeg-decoder.txt to yaml, but
missed to adjust its reference in MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference.

While touching this section, generalize the file entry to also cover the
encoder yaml devicetree, as the driver directory also includes the encoder.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>