regulator: documentation - regulator driver

This adds documentation describing the regulator driver interface.
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>

Documentation/power/regulator/regulator.txt

+Regulator Driver Interface
+==========================
+
+The regulator driver interface is relatively simple and designed to allow
+regulator drivers to register their services with the core framework.
+
+
+Registration
+============
+
+Drivers can register a regulator by calling :-
+
+struct regulator_dev *regulator_register(struct regulator_desc *regulator_desc,
+					  void *reg_data);
+
+This will register the regulators capabilities and operations the regulator
+core. The core does not touch reg_data (private to regulator driver).
+
+Regulators can be unregistered by calling :-
+
+void regulator_unregister(struct regulator_dev *rdev);
+
+
+Regulator Events
+================
+Regulators can send events (e.g. over temp, under voltage, etc) to consumer
+drivers by calling :-
+
+int regulator_notifier_call_chain(struct regulator_dev *rdev,
+				  unsigned long event, void *data);