Add an entry for imx27-pdk.dtb, so that it can be generated by default.

Also, add an entry into Documentation/devicetree/bindings/arm/fsl.txt.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>