mfd: Add power off functionality to TWL

TWL family of PMICs, used in master mode, have a power off
functionality. The resulting power off sequence shuts down all the SoC
supplies, LDOs, etc. The sequence is described in the datasheets
chapter "Power-Off Sequence".
Note, that board must be wired correctly for the power off to work as
expected.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>