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>