arm64: dts: rockchip: add phandles to muxed i2c buses on rk3368-lion

Other DTS files that include the dtsi will want to to add children
to the i2c buses from the i2c-mus. Without a label they would have to
specify the full path.

Signed-off-by: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
[add phandles for first mux as well]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20210922230429.2162535-3-heiko@sntech.de