board/BuR/tseries: Add simplefb support

Since the used AM3352 SoC doesn't have GPU it isn't allways necessary to build
in complete drm-stuff into linux kernel. In very small applications only we use
the simple-framebuffer.

So we have 2 use-cases:
- device operating on drm-driver (let simplefb node disabled)
- device operating on simplefb-driver (activate simplefb node and reserve mem)

The decision is made by means of "simplefb" environment variable.

simplefb = 0
we don't enable the (maybe) existing simplefb node and all the rest around
display is up to the linux-kernel. We just disable the backlight, beceause we
do not want see the flicker during take over of drm-driver.

simplefb = 1
we enable the (maybe) existing simplefb node and reserve framebuffers size
in memory.

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>