arm64: Add a CPU reset hook instead of expecting PSCI

[stevek]

Summary

Some SoCs do not include a PSCI for power management and defer it to
something else instead. Add a CPU reset hook to account for this, and
use it in the psci driver.

Obtained from: Juniper Networks, Inc.

[andrew]

Is there a reason to not have cpu_reset_hook = psci_reset by default so we can remove the NULL check?

[stevek]

In D44535#1015791, @andrew wrote:

Is there a reason to not have cpu_reset_hook = psci_reset by default so we can remove the NULL check?

Given that psci.c is "standard" on arm64 and it will not do anything until/unless it probes/attaches (and thus psci_softc is non-NULL), yes, I believe it could just default to psci_reset (maybe with a comment stating the reasons why?)

[stevek]

Updated based on review comments