OMAP: DSS2: DSI Video mode support

Add initial support for DSI video mode panels:
- Add a new structure omap_dss_dsi_videomode_data in the member "panel" in
  omap_dss_device struct. This allows panel driver to configure dsi video_mode
  specific parameters.
- Configure basic DSI video mode timing parameters: HBP, HFP, HSA, VBP, VFP, VSA,
  TL and VACT.
- Configure DSI protocol engine registers for video_mode support.
- Introduce functions dsi_video_mode_enable() and dsi_video_mode_disable() which
  enable/disable video mode for a given virtual channel and a given pixel format
  type.

Things left for later
- Add functions to check for errors in video mode timings provided by panel.
- Configure timing registers required  for command mode interleaving.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>