media: MAINTAINERS: Add myself as Venus reviewer

I'd like to be looped in on changes to venus so that I can help ensure no
obvious regressions creep in. I'm happy to run some simple tests for venus
on the hardware I have access to

- db410c
- db820
- rb3
- rb5

and one of rb1 / rb2 in the future when they are delivered.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>