arm64: dts: rockchip: add DDC bus on Rock Pi 4

A DDC I2C bus specifier is required for DDC EDID probing to work
properly.

Fixes: 1b5715c6 ("arm64: dts: rockchip: add ROCK Pi 4 DTS support")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>