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>