extcon: Simplify extcon_dev_register() prototype by removing unnecessary parameter

This patch remove extcon_dev_register()'s second parameter which means
the pointer of parent device to simplify prototype of this function.
So, if extcon device has the parent device, it should set the pointer of
parent device to edev.dev.parent in extcon device driver instead of in
extcon_dev_register().

Cc: Graeme Gregory <gg@slimlogic.co.uk>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>