Add RK3576 SAI Audio Controller Support

Merge series from Nicolas Frattaroli <nicolas.frattaroli@collabora.com>:

This series adds support for Rockchip's Serial Audio Interface (SAI)
controller, found on SoCs such as the RK3576. The SAI is a flexible
controller IP that allows both transmitting and receiving digital audio
in the I2S, TDM and PCM formats. Instances of this controller are used
both for externally exposed audio interfaces, as well as for audio on
video interfaces such as HDMI.