video: fbdev: pxafb: initial devicetree conversion

This patch brings a first support of pxa framebuffer devices to a
devicetree pxa platform, as was before platform data.

There are restrictions with this port, the biggest one being the lack of
support of smart panels. Moreover the conversion doesn't provide a way
to declare multiple framebuffer configurations with different bits per
pixel, only the LCD hardware bus width is used.

The patch was tested on both pxa25x, pxa27x and pxa3xx platform (namely
lubbock, mainstone and zylonite).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>