ARM: restart: orion5x: use new restart hook

Hook these platforms restart code into the new restart hook rather than
using arch_reset().

In addition, convert calls to arm_machine_restart() to orion5x_restart()
to ensure that they continue to work as intended.

Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>