arm: vexpress: Decouple vexpress-poweroff implementation from machine_desc

This patch adds the pm_power_off and arm_pm_restart variable settings to
the vexpress-poweroff.c driver to decouple it from the machine_desc
definition.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>