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We simulate the PSCI CPU_OFF operation by reseting the core via RMR.
For secondaries, that already puts them in the holding pen waiting for a
"warm boot" request as part of PSCI CPU_ON. For the BSP, we have to add
logic to distinguish a regular boot from a CPU_OFF state, where, like the
secondaries, the BSP needs to wait foor a "warm boot" request as part
of CPU_ON.

Testing done:

- ACS suite now passes more tests (since it repeatedly
calls code on secondaries via CPU_ON).

- Linux testing including offlining/onlineing CPU0, e.g.
"echo 0 > /sys/devices/system/cpu/cpu0/online".

Change-Id: Id0ae11a0ee0721b20fa2578b54dadc72dcbd69e0
Link: https://developer.trustedfirmware.org/T686

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
[Andre: adapt to unified plat_helpers.S, smaller fixes]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>