regulator: arizona: Add regulator specific device tree binding document

Subsystem specific bindings for the Arizona devices are being factored
out of the MFD binding document into separate documents for each
subsystem. This patch adds a binding document that covers the existing
regulator specific bindings.
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/arizona-regulator.txt

+Cirrus Logic Arizona class audio SoCs
+
+These devices are audio SoCs with extensive digital capabilities and a range
+of analogue I/O.
+
+This document lists regulator specific bindings, see the primary binding
+document:
+  ../mfd/arizona.txt
+
+Optional properties:
+  - wlf,ldoena : GPIO specifier for the GPIO controlling LDOENA
+
+Optional subnodes:
+  - ldo1 : Initial data for the LDO1 regulator, as covered in
+    Documentation/devicetree/bindings/regulator/regulator.txt
+  - micvdd : Initial data for the MICVDD regulator, as covered in
+    Documentation/devicetree/bindings/regulator/regulator.txt