kernel/reboot.c: export pm_power_off_prepare

Export pm_power_off_prepare. It is needed to implement power off on
Freescale/NXP iMX6 based boards with external power management
integrated circuit (PMIC).

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>