dt-bindings: extcon: Add support for Richtek RT8973A MUIC device

This patch add documentation for binding of Richtek RT8973A (Micro USB Switch)
device which is using EXTCON subsystem. The RT8973A device can detect various
external accessories when external accessories is attached or detached.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>