dt-bindings: extcon: bindings for SM5703

This patch adds device-tree bindings for Silicon Mitus SM5703 MUIC.

Signed-off-by: Markuss Broks <markuss.broks@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>