ARMv8: Add support for poweroff via PSCI

Add support for calling poweroff in case of psci is wired.
Based on the same solution as is used for reset.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Move all logic in to fwcall.c as other ARMs implement poweroff
via PMIC]
Signed-off-by: Tom Rini <trini@konsulko.com>