extcon: arizona: Add extcon 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
extcon specific bindings.
Signed-off-by: NCharles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: NChanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/extcon/extcon-arizona.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 Extcon specific bindings, see the primary binding document:
+  ../mfd/arizona.txt
+
+Optional properties:
+
+  - wlf,hpdet-channel : Headphone detection channel.
+    ARIZONA_ACCDET_MODE_HPL or 1 - Headphone detect mode is set to HPDETL
+    ARIZONA_ACCDET_MODE_HPR or 2 - Headphone detect mode is set to HPDETR
+    If this node is not mentioned or if the value is unknown, then
+    headphone detection mode is set to HPDETL.