spi: fsl-lpspi: Add imx8ulp compatible string

The lpspi on i.MX8ULP is derived from i.MX7ULP, it uses two
compatible strings, so update the comaptible string for i.MX8ULP.
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211120113454.785997-4-peng.fan@oss.nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-fsl-lpspi.yaml

@@ -14,10 +14,13 @@ allOf:
 
 properties:
   compatible:
-    enum:
-      - fsl,imx7ulp-spi
-      - fsl,imx8qxp-spi
-
+    oneOf:
+      - enum:
+          - fsl,imx7ulp-spi
+          - fsl,imx8qxp-spi
+      - items:
+          - const: fsl,imx8ulp-spi
+          - const: fsl,imx7ulp-spi
   reg:
     maxItems: 1