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 <[e-mail]>
Reviewed-by: Heiko Stuebner <[e-mail]>
Signed-off-by: Yao Zi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml

···
               - rockchip,rk3328-uart
               - rockchip,rk3368-uart
               - rockchip,rk3399-uart
+              - rockchip,rk3528-uart
               - rockchip,rk3568-uart
               - rockchip,rk3576-uart
               - rockchip,rk3588-uart