Add initial clocks support for Rockchip RK3568 SoC.

Partially from: https://reviews.freebsd.org/D36027

This can be eventually improved or simplified or fixed if necessary.

Following devices work with proper drivers and with the necessary clocks:
Native networking via eqos driver
USB3 and USB2
PCIe support is working but a bit picky about what hardware it supports (but so is Linux)
SD & (e)MMC
With the EDK2 loader video also works

Supported hardwares are Quartz64, NanoPI R5S and Firefly Station P2, more to come as DTS files gets done.