dt-bindings: i2c: i2c-rk3x: add description for rk3568

add "rockchip,rk3568-i2c", "rockchip,rk3399-i2c" for i2c nodes on
a rk3568 platform to i2c-rk3x.yaml.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>