of: documentation: add bindings documentation for TS-4800

This adds the documentation for the TS-4800 by Technologic Systems.
Signed-off-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/technologic.txt

+Technologic Systems Platforms Device Tree Bindings
+--------------------------------------------------
+
+TS-4800 board
+Required root node properties:
+	- compatible = "technologic,imx51-ts4800", "fsl,imx51";