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