ARM: OMAP3+: SmartReflex: Add a shutdown hook

SmartReflex fix for erratum ID i724.

Since OMAP's VP and PRM modules do not get reset by warm reset,
we should ensure that proper shutdown procedure is followed prior
to allowing the kernel to reboot back up.

Without this, Smartreflex module might be left active or
system might be caught in an indeterminate sequence when
software controlled reboot is triggered, leaving the next
reboot behavior to be unpredictable.

In the case of hardware controlled warm reset such as that
by watchdog timer, prevention of this scenario is not possible.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>