spi: dt-bindings: cadence: add missing 'required'

During the conversion the bindings lost list of required properties.

Fixes: aa796868 ("spi: convert Cadence SPI bindings to YAML")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20220704130618.199231-1-krzysztof.kozlowski@linaro.org


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