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

new compatible string: "fsl,ls1088a-dspi".

Signed-off-by: Chuanhua Han <chuanhua.han@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>