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

In doing so, we split out the n2100 platform specific restart handler
into the n2100 platform file.
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>