dt-bindings: serial: snps-dw-apb-uart: correct number of DMAs

The "minItems" alone does not impose any upper limit of DMAs, so switch
it to "maxItems" which also implies same value for minItems.

Fixes: 370f696e

 ("dt-bindings: serial: snps-dw-apb-uart: add dma & dma-names properties")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230317155712.99654-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>