arm: minimal PSCI implementation

arm: minimal PSCI implementation

This is not the final thing: it can only be used internally for
suspending, resuming and parking CPUs while reconfiguring the cells.
Using this base, a trivial PSCI 0.2 emulation can be added by implementing
the appropriate trap hooks.

The mailbox in the per_cpu structure is used to store the address and
context where psci_cpu_off returns.
CPUs doing reconfiguration on the cells can use the psci_suspend and
psci_resume wrappers to store all affected cores.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>