[media] davinci: vpif: make generic changes to re-use the vpif drivers on da850/omap-l138 soc

change the dm646x specific strings in the driver to make
them generic across platforms. In this case change all the
strings which have a dm646x connotation to vpif which is a
platform independent ip.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>