mfd: Add device tree support for max77686

This patch adds device tree support for mfd driver and adds
Documentation/devicetree/bindings/mfd/max77686.txt.
This patch also intialize max77686 pointer to NULL in max77686_i2c_probe
to silent a compile time warning.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>