ASoC: rockchip: spdif: Cleanups and port features

Merge series from Sebastian Reichel <sebastian.reichel@collabora.com>:

I'm currently working on DisplayPort audio support for the Rockchip
RK3588/RK3576 SoCs, which preferrably use S/PDIF as DAI source.

Apparently the upstream Rockchip S/PDIF driver is lacking a couple of
features right now, which are necessary to get things going (i.e.
setting the sysclk from the machine driver). I found the missing bits
in Rockchip's 6.1 BSP kernel and ported them over. This series effectly
brings the mainline kernel on-par with the BSP driver, but also contains
a couple of cleanup patches of my own to bring the driver to the modern
age.