dt-bindings: mmc: rockchip-dw-mshc: add description for rk3308

The description below is already in use for rk3308.dtsi,
but was somehow never added to a document, so add
"rockchip,rk3308-dw-mshc", "rockchip,rk3288-dw-mshc"
for mmc nodes on a rk3308 platform to rockchip-dw-mshc.yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200116152230.29831-3-jbx6244@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
diff --git a/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.yaml b/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.yaml
index 2f70f5e..89c3edd 100644
--- a/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.yaml
+++ b/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.yaml
@@ -35,6 +35,8 @@
             - rockchip,rk3036-dw-mshc
             # for Rockchip RK322x
             - rockchip,rk3228-dw-mshc
+            # for Rockchip RK3308
+            - rockchip,rk3308-dw-mshc
             # for Rockchip RK3328
             - rockchip,rk3328-dw-mshc
             # for Rockchip RK3368