dt-bindings: serial: snps-dw-apb-uart: make interrupt optional

The driver fall back to poll style when there's no irq. "poll" still
looks better than no support.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230806092056.2467-2-jszhang@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>