dev/psci: Add initial SDEI support

[andrew]

Summary

Add support for the Arm Software Delegated Exception Interface. This
is an interface that allows us to receive exceptions from firmware.

Currently only event 0 is supported. This is a software generated event
that is used to implement a non-maskable IPI. This is used by kdb to
ensure other CPUs are halted before entering the debugger. As real
non-maskable interrupts are a recent addition to the architecture SDEI
can be used to emulate them on existing hardware that supports it in
the firmware.

Other events are not yet supported, but could be added.

Sponsored by: Arm Ltd