video: Allow selection of the driver and font size

Provide a way for the video console driver to be selected. This is
controlled by the video driver's private data. This can be set up when the
driver is probed so that it is ready for the video_post_probe() method.

The font size is provided as well. The console driver may or may not support
this depending on its capability.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>