spi: spi-fsl-lpspi: add num-cs binding for lpspi

Add num-cs property to support multiple cs for lpspi. This property is
optional.

Signed-off-by: Han Xu <han.xu@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221206225410.604482-2-han.xu@nxp.com


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