cdns,page-size and cdns,block-size are neither documented nor used by
Linux, so remove them to fix dtbs_check warnings like:

  socfpga_n5x_socdk.dtb: flash@0: Unevaluated properties are not allowed ('cdns,block-size', 'cdns,page-size' were unexpected)
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>