mfd: as3711: Add OF support

Add Flat Device Tree support to the AS3711 MFD driver. This patch just
allows to bind the driver to I2C devices, instantiated from the DT.
DT support for AS3711 cell drivers will be added in separate drivers.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>