ARM: vexpress/TC2: implement PM suspend method

Similar to power_down(), except that for a suspend, the firmware
mailbox address has to be set prior entering low power mode.

The residency argument is not used yet, so the last man always shuts
down the cluster for now.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Pawel Moll <pawel.moll@arm.com>