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: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210429081151.17558-3-cl@rock-chips.com


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