fdt_support: Add a fdt_setup_simplefb_node helper function

Add a generic helper to fill and enable simplefb nodes.

The first user of this will be the sunxi display code.

lcd_dt_simplefb_configure_node is also a good candidate to be converted
to use this, but that requires someone to run some tests first, as
lcd_dt_simplefb_configure_node does not honor #address-cells and #size-cells,
but simply assumes 1 and 1 for both.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Simon Glass <sjg@chromium.org>