dt-bindings: serial: snps-dw-apb-uart: add description for rk3568

add "rockchip,rk3568-uart", "snps,dw-apb-uart" for uart nodes on
a rk3568 platform to snps-dw-apb-uart.yaml.
Signed-off-by: NLiang Chen <cl@rock-chips.com>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210429081151.17558-3-cl@rock-chips.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -31,6 +31,7 @@ properties:
               - rockchip,rk3328-uart
               - rockchip,rk3368-uart
               - rockchip,rk3399-uart
+              - rockchip,rk3568-uart
               - rockchip,rv1108-uart
           - const: snps,dw-apb-uart
       - items: