MAINTAINERS: add an explicit entry for cx88

While this file was always maintained, there was no explicit entry
for it.

There aren't many things happening at cx88 side for a long time:
Driver works, and it is stable. When new patches are submitted,
they're new boards addition for hardware I don't have.

So, add a new entry with status equal to Odd fixes to better
reflect it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>