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.be
Signed-off-by: Mark Brown <broonie@kernel.org>