dt-bindings: spi: Add fsl,ls1012a-dspi compatible string

new compatible string: "fsl,ls1012a-dspi".

Signed-off-by: Yuan Yao <yao.yuan@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>