mfd: max8907: Add power off control

Add DT property "maxim,system-power-controller" to indicate whether the
PMIC is in charge of controlling the system power. If this is set, the
driver will provide the pm_power_off() function.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>