Implement the fundamentals for suspend/resume on arm64. This uses PSCI

Implement the fundamentals for suspend/resume on arm64.  This uses PSCI
to turn off the secondary CPUs and suspend the primary CPU using the
CPU_OFF and SYSTEM_SUSPEND calls.  A new "halt" IPI is added to turn off
the ssecondary CPUs.  This IPI is implemented for the ampintc(4) and
agintc(4) interrupt controllers.  Fulle suspend/resume support is only
implemented for ampintc(4).  This is enough to suspend and resume boards
based on the Allwinner A64 SoC, provided the necessary wakeup interrupts
have been set up (not part of this commit).

ok patrick@