dt-bindings: i2c: i2c-rk3x: add description for px30

The description below is already in use for px30.dtsi,
but was somehow never added to a document, so add
"rockchip,px30-i2c", "rockchip,rk3399-i2c"
for i2c nodes on a px30 platform to i2c-rk3x.yaml.

Signed-off-by: Johan Jonker <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

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

···
           - const: rockchip,rk3288-i2c
       - items:
           - enum:
+            - rockchip,px30-i2c
             - rockchip,rk3308-i2c
             - rockchip,rk3328-i2c
           - const: rockchip,rk3399-i2c