dt-bindings: extcon: add binding for TUSB320

Add a device tree binding for the TI TUSB320.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>