power: reset: imx-snvs-poweroff: add power off driver for i.mx6

This driver register pm_power_off with snvs power off function. If
your boards NOT use PMIC_ON_REQ to turn on/off external pmic, or use
other pin to do, please disable the driver in dts, otherwise, your
pm_power_off maybe overwrote by this driver.

Signed-off-by: Robin Gong <b38343@freescale.com>
Acked-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>