dt-bindings: serial: snps-dw-apb-uart: Add Rockchip RK1808

Add a Rockchip RK1808 compatible.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210516230551.12469-3-afaerber@suse.de


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