ARM: bcm2835: add simplefb DT node during bootz/m

Add a DT simple-framebuffer node to DT when booting the Linux kernel.
This will allow the kernel to inherit the framebuffer configuration from
U-Boot, and display a graphical boot console, and even run a full SW-
rendered X server.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Simon Glass <sjg@chromium.org>