spi: spi-fsl-dspi: Update DT binding documentation

DSPI driver has been updated and support more compatible strings.
This patch update the DT binding documentation.
Signed-off-by: Haikun Wang <haikun.wang@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-fsl-dspi.txt

 ARM Freescale DSPI controller
 
 Required properties:
-- compatible : "fsl,vf610-dspi"
+- compatible : "fsl,vf610-dspi", "fsl,ls1021a-v1.0-dspi", "fsl,ls2085a-dspi"
 - reg : Offset and length of the register set for the device
 - interrupts : Should contain SPI controller interrupt
 - clocks: from common clock binding: handle to dspi clock.