extcon: Move defintion of struct extcon_dev to driver/extcon directory

This patch moves the 'struct extcon_dev' of extcon subsystem
to driver/extcon/extcon.h header file because the struct extcon_dev have to
be handled by extcon API to guarantee the consistency of strcut extcon_dev.
If external drivers are able to touch the struct extcon_dev directly, it might
cause the critical and unknown problem.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>