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>

diff --cc drivers/extcon/Kconfig
Simple merge

diff --cc drivers/extcon/Makefile
Simple merge

diff --cc drivers/extcon/extcon-class.c
Simple merge

diff --cc drivers/extcon/of_extcon.c
Simple merge

diff --cc drivers/usb/dwc3/dwc3-omap.c
Simple merge

diff --cc include/linux/extcon.h
Simple merge

diff --cc include/linux/extcon/of_extcon.h
Simple merge