ARM: dts: rockchip: move the hdmi ddc-i2c-bus property to the actual boards

Currently the hdmi driver is using one of the soc i2c busses for ddc probing
and while documentation always specifies i2c5 as hdmi-i2c it could very well
be any other bus as well.

Therefore this is a property of the board and should be specified there.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>