arm: implement the cell destruction

arm: implement the cell destruction

Give the CPUs back to Linux when a cell is destroyed, after resetting its
whole context.

This patch uses the old vexpress CPU hotplug system in Linux: the
secondary startup function address is kept in the system flags, so the
cpu_reset function will simply jump there after resetting the CPU state.
A future patch will add PSCI hotplug support, and once the hypervisor
has access to a device tree, this spin function will need to be set
dynamically in a `struct hotplug_ops'.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: switch to mmio accessors]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>