OMAPDSS: VENC: allow switching venc output type at runtime

VENC output type (composite/svideo) doesn't have to be fixed by board
wiring, it is possible to also provide composite signal through svideo
luminance connector (software enabled), which is what pandora does.

Having to recompile the kernel for users who have TV connector types
that don't match default board setting is very inconvenient, especially
for users of a consumer device, so add support for switching VENC output
type at runtime over a new sysfs file output_type.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>