fdt_support: support adding EDID property to FDT display nodes

Boards can pass display timing info for drivers using EDID
block. Provide common function to add board specific EDID
data to the device tree. Subsequent patch makes use of this
functionality.

Detailed timing descriptor data from EDID is used for
programming the display controller. This is currently
implemented on the Linux side by the fsl-diu-fb frame
buffer driver and it is documented there in
Documentation/powerpc/dts-bindings/fsl/diu.txt.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>
Cc: Gerald Van Baren <vanbaren@cideas.com>