MAINTAINERS: update email and git tree

While mchehab@infradead.org is valid, I prefer to use just one email
for all patches upstream, instead of receiving some things on one
emails, and others on some other place.

While here, also update the main linux-media development tree,
as the one at kernel.org is used mainly for patch merging and it
is generally delayed.

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