ARM: imx6: register pm_power_off handler if "fsl,pmic-stby-poweroff" is set

commit 8148d213

 upstream.

One of the Freescale recommended sequences for power off with external
PMIC is the following:
...
3.  SoC is programming PMIC for power off when standby is asserted.
4.  In CCM STOP mode, Standby is asserted, PMIC gates SoC supplies.

See:
http://www.nxp.com/assets/documents/data/en/reference-manuals/IMX6DQRM.pdf
page 5083

This patch implements step 4. of this sequence.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
[xl: Just register pm_power_off handler if "fsl,pmic-stby-poweroff" is
set since commit 57b59efd

("ARM: imx: add suspend/resume with FastMix
OFF support") has backported imx6_pm_stby_poweroff_probe and
imx6_pm_stby_poweroff.]
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>