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

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

[ Upstream commit 8148d2136002da2e2887caf6a07bbd9c033f14f3 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>