mfd: dt: tps6586x: Add power off control

Add DT property "ti,system-power-controller" telling whether or not this
pmic is in charge of controlling the system power, so the power off
routine can be hooked up to system call "pm_power_off".

Based on the work by:
Dan Willemsen <dwillemsen@nvidia.com>

Signed-off-by: Bill Huang <bilhuang@nvidia.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>