Extend the vpif node with an output port with a single channel.

NOTE: this is still mostly just hardware description - the actual
driver is registered using pdata-quirks. We need the node however
for correct pin control function selection.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>