arm: implement hypervisor shutdown

arm: implement hypervisor shutdown

When an HV_DISABLE hypercall is issued on all root CPUs by the driver,
the core `shutdown' function executes the following operations:
- Suspend all non-root cells (all the CPUs are taken to hyp idle mode),
- call arch_shutdown_cpu for all those CPUs,
- call arch_shutdown.
Once the master CPU (the first to take the shutdown lock) did this, the
other root CPUs don't actually perform any operation.

This patch lets the arch_shutdown and arch_shutdown_cpu set a boolean
that is considered by the cores right before returning to EL1: for the
cells' CPUs, arch_shutdown_cpu will trigger a return to arch_reset_self,
that will clean up EL1 and EL2. On the root cpus, the exit handler
checks this boolean and calls the shutdown function.

Once inside arch_shutdown_self, the principle is the same as with the
hypervisor initialisation:
- Create identity mappings of the trampoline page and the stack,
- Jump to the physical address of the shutdown function,
- Disable the MMU,
- Reset the vectors,
- Return to EL1

This patch does not handle hosts using PSCI yet: they will need to issue
a final SMC on secondary CPUs in order to park themselves at EL3, since
the hypervisor won't exist anymore to emulate the wakeup call.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: moved arch_shutdown_cpu & arch_shutdown to control.c]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>