mfd: palmas: Add power off control

Hook up "pm_power_off" to palmas power off routine if there is DT
property "ti,system-power-controller" defined, so platform which is
powered by this regulator can be powered off properly.

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mallikarjun Kasoju <mkasoju@nvidia.com>
Signed-off-by: Bill Huang <bilhuang@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>