[media] tm6000: Add support for Terratec Grabster AV 150/250 MX

This patch adds support for Terratec Grabster AV 150/250 MX. For now it is
only possible to use composite input as switching inputs does not work.

[mchehab@redhat.com: Fix merge conflicts and simplify board entries]
Signed-off-by: Holger Nelson <hnelson@hnelson.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>