arm64: dts: rockchip: Add core dts for RK3308 SOC

RK3308 is a quad Cortex A35 based SOC with rich audio
interfaces(I2S/PCM/TDM/PDM/SPDIF/VAD/HDMI ARC), which
designed for intelligent voice interaction and audio
input/output processing.

This patch add basic core dtsi file for it.
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Link: https://lore.kernel.org/r/20191021084616.28431-1-andy.yan@rock-chips.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>