lcd: add functions to set up simplefb device tree

simple-framebuffer is a new device tree binding that describes a pre-
configured frame-buffer memory region and its format. The Linux kernel
contains a driver that supports this binding. Implement functions to
create a DT node (or fill in an existing node) with parameters that
describe the framebuffer format that U-Boot is using.

This will be immediately used by the Raspberry Pi board in U-Boot, and
likely will be used by the Samsung ARM ChromeBook support soon too. It
could well be used by many other boards (e.g. Tegra boards with built-in
LCD panels, which aren't yet supported by the Linux kernel).

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