cpufreq: rockchip: Introduce driver for rk3588

This is a heavily modified port from the downstream driver.
Downstream used it for multiple rockchip generations, while
upstream just used the generic cpufreq-dt driver so far. For
rk3588 this is no longer good enough, since two regulators
need to be controlled.

Also during shutdown the correct frequency needs to be configured
for the big CPU cores to avoid a system hang when firmware tries
to bring them up at reboot time.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>