dt-bindings: lpspi: Add missing boolean type for fsl,spi-only-use-cs1-sel

When running "make dt_binding_check" (even if restricted to an unrelated
binding document using DT_SCHEMA_FILES=...):

    Documentation/devicetree/bindings/spi/spi-fsl-lpspi.yaml: ignoring, error in schema: properties: fsl,spi-only-use-cs1-sel
    warning: no schema found in file: Documentation/devicetree/bindings/spi/spi-fsl-lpspi.yaml

Fix this by adding a proper type definition for the vendor-specific
fsl,spi-only-use-cs1-sel property.

Fixes: 7ac9bbf6 ("dt-bindings: lpspi: New property in document DT bindings for LPSPI")
Suggested-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200807121057.14204-1-geert+renesas@glider.beSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -39,6 +39,7 @@ properties:
       spi common code does not support use of CS signals discontinuously.
       i.MX8DXL-EVK board only uses CS1 without using CS0. Therefore, add
       this property to re-config the chipselect value in the LPSPI driver.
+    type: boolean
 
 required:
   - compatible