ARM: restart: add restart hook to machine_desc record

Add a restart hook to the machine_desc record so we don't have to
populate all platforms with init_early methods to initialize the
arm_pm_restart function pointer.

Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>