dt-bindings: i2c: i2c-rk3x: 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-i2c", "rockchip,rk3399-i2c"
for i2c nodes on a rk3308 platform to i2c-rk3x.yaml.
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/i2c/i2c-rk3x.yaml

@@ -33,6 +33,7 @@ properties:
           - const: rockchip,rk3288-i2c
       - items:
           - enum:
+            - rockchip,rk3308-i2c
             - rockchip,rk3328-i2c
           - const: rockchip,rk3399-i2c