ARM: restart: remove comments about adding code to arch_reset()

arch_reset() is deprecated; systems should hook into system restart via
the 'restart' method in the platforms machine description record.

Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>