OMAPDSS: add fields to panels' platform data

Forthcoming panel patches will change the panel drivers to use platform
data to pass panel's gpios to the panel driver. This patch adds the
required fields and platform data structs to the omap-panel-data.h file,
so that the board files can be changed independently of the panel driver
changes.

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