ARM: restart: at91: use new restart hook

Rather than using a private function pointer, use the existing
arm_pm_restart function pointer instead.  We no longer need to enable
the I-cache in at91sam9_alt_reset() as the caches will now be on when
this function is called.

Update the function names to use the 'restart' terminology rather than
the 'reboot' terminology.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>