RK3588 SoC subsystem performance counters

RK3588 has hardware to measure bus bandwidth used by different subsystems of the SoC. Mainline can currently measure the overall used memory bandwidth with drivers/devfreq/event/rockchip-dfi.c, but the per-SoC-subsystem bus bandwidth counters are not yet implemented.

Especially for tasks like video capture from sensors, having access to these perf counters would be nice for debugging.

• LPDDR5 support for existing rockchip-dfi driver
• Identify what downstream code is used for this, determine its state
• Port/write an upstream solution
• Document how to make use of them somewhere other than the patch series cover letter