dt-bindings: mmc: rockchip-dw-mshc: Add Rockchip RK1808

Add a compatible string for Rockchip RK1808 SoC.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210516230551.12469-7-afaerber@suse.de


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>