dt-bindings: serial: snps-dw-apb-uart: Document Rockchip RK3528

Rockchip RK3528 comes with a snps-dw-apb-uart compatible UART. Document
it in dt-bindings.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yao Zi <ziyao@disroot.org>
Link: https://lore.kernel.org/r/20240919111413.45413-4-ziyao@disroot.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>