extcon: Add stubs for extcon_register_notifier_all() functions

Add stubs for extcon_register_notifier_all() function for !CONFIG_EXTCON
case.  This is useful for compile testing and for drivers which use
EXTCON but do not require it (therefore do not depend on CONFIG_EXTCON).

Fixes: 815429b3 ("extcon: Add new extcon_register_notifier_all() to monitor all external connectors")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>