RK3576 DisplayPort

According to the mainline-status matrix for RK3576, "DP is very similar to RK3588".

RK3588 DP is currently being mainlined by Andy Yan.

We should probably wait with getting started on this until the RK3588 series is applied to -next.

• Wait for RK3588 series to be merged
• Implement RK3576 support in driver and SoC dtsi
• (prereq: Sige5 USB series) enable USB-C DP alt-mode on Sige5
• Get it into linux-next