mfd: Add device-tree support for da9502 i2c driver

This patch adds device-tree support for dialog MFD and the binding
documentations.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Ashish Jangam <ashish.jangam@kpitcummins.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>