extcon: Add an API to get extcon device from dt node

Added an API of_extcon_get_extcon_dev() to be used by drivers to get
extcon device in the case of dt boot (this can be used instead of
extcon_get_extcon_dev()).

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>