Support the camera connector on the OSK Mistral add-on board:

  - define muxing for both camera controllers
  - mux both of them for Mistral
  - teach ov9640 glue about mistral powerup/powerdown
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>