V4L/DVB (13163): Docbook/dvb: Rename isdbt.xml to dvbproperty.xml

We'll need to add the documentation for FE_[GET|SET]_PROPERTY that
is not specific to isdb-t. Instead of doing it at frontend.xml,
it is better to have a separate session for it. Also, isdbt.xml assumes
that those ioctls were already introduced, and documents several exisiting
properties. So, better to just rename it and add the documentation for it,
together with each existing property there.

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