arm64: dts: rockchip: Enable HDMI audio devices on rk3399-rock960

This commit enable the hdmi-sound and i2s2 devices needed to have
audio over HDMI on both rock960 and the related ficus board.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>