extcon: Move OF helper function to extcon core and change function name

This patch move simply OF helper function to extcon core and change function
name as following:
- of_extcon_get_extcon_dev() -> extcon_get_edev_by_phandle()

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Felipe Balbi <balbi@ti.com>