dt-bindings: fsl: dspi: Add fsl,ls1088a-dspi compatible string

new compatible string: "fsl,ls1088a-dspi".
Signed-off-by: NChuanhua Han <chuanhua.han@nxp.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

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

@@ -6,6 +6,7 @@ Required properties:
 		or
 		"fsl,ls2080a-dspi" followed by "fsl,ls2085a-dspi"
 		"fsl,ls1012a-dspi" followed by "fsl,ls1021a-v1.0-dspi"
+		"fsl,ls1088a-dspi" followed by "fsl,ls1021a-v1.0-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.