ipq401x: add MP core start-up path for the CPU regulator/clock gate used

This code implements the "kpssv2" flavour of CPU regulator/clock gating
in Linux. It's used by at least the ipq4018/4019 to power on and off
CPU cores.

This is based on the Linux implementation - the register definitions
and values are from Linux and I've reverse engineered the sequencing
requirements.

The MP bring-up is:

• set cold boot address via an SCM call - this is the address used by the bootloader/TZ firmware to jump to when the CPUs boot

• power down the LDO feeding the CPU core and wait for it to settle

• program in the right set of LDO and power tree configuration for the CPU regulator to power up the core. Unfortunately these are magic numbers that I've not found documented anywhere.

• (I think) power up the shared L2 cache connect if it isn't.

• Clamp the power into the core down; put the core into reset

• Unclamp the power rail; release reset; and then set the core to boot.

The MP core will then boot the bootloader/TZ firmware and then
will wait until an incoming interrupt kicks it to start @ mpentry.

Tested:

• IPQ4019, 4 CPUs

Release APs
CPU(3) applied BP hardening: not necessary
CPU(1) applied BP hardening: not necessary
CPU(2) applied BP hardening: not necessary

Reviewed by: andrew, manu, imp
Differential Revision: https://reviews.freebsd.org/D32723