firmware: arm_sdei: add support for CPU private events

Private SDE events are per-cpu, and need to be registered and enabled
on each CPU.

Hide this detail from the caller by adapting our {,un}register and
{en,dis}able calls to send an IPI to each CPU if the event is private.

CPU private events are unregistered when the CPU is powered-off, and
re-registered when the CPU is brought back online. This saves bringing
secondary cores back online to call private_reset() on shutdown, kexec
and resume from hibernate.

Signed-off-by: James Morse <james.morse@arm.com>

CC: bogus bogus bogus

Changes since v4:
 * rip out the racy frozen flag.
 * hotplug hooks honour the event reregister/renable flags instead of
   save/restoring the firmware state, these flags are protected by the
   spinlock.
 * Repurposed the event-enable ipi-call instead of having a redundant
   re-enable call.
 * Dropped Catalin's ack