mfd: twl6040: Add twl6040-gpio child

Add needed platform data structure and code to be able to load
the GPO child of twl6040.
Update the devicetree binding documentation at the same time.

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>