video: add nexell video driver (display/video driver)

Changes in relation to FriendlyARM's U-Boot nanopi2-v2016.01:
- nexell_display.c: Changed to DM, CONFIG_FB_ADDR can not be used
  anymore because framebuffer is allocated by video_reserve() in
  video-uclass.c. Therefore code changed appropriately.
- '#ifdef CONFIG...' changed to 'if (IS_ENABLED(CONFIG...))' where
  possible (and similar).
- livetree API (dev_read_...) is used instead of fdt one (fdt...).

Signed-off-by: Stefan Bosch <stefan_b@posteo.net>