sun7i: Add support for taking CPUs offline via PSCI

Based on the original version by Marc Zyngier. It adds a psci_cpu_off
implementation for the A20 SoC. The mechanism works by first preparing
the calling CPU to go offline (disable and flush cache, disable SMP),
then requesting CPU 0 to pull the plug. The request is sent as FIQ on
SGI15.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>