video: omap: add panel driver

The previous version of am335x-fb.c contained the functionalities of two
drivers that this patch has split. It was a video type driver that used
the same registration compatible string that now registers a panel type
driver. The proof of this is that two compatible strings were referred
to within the same driver.
There are now two drivers, each with its own compatible string,
functions and API.
Furthermore, the panel driver, in addition to decoding the display
timings, is now also able to manage the backlight.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>