Extcon: add MAX8997 extcon driver

This patch add extcon-max8997 driver to support the muic feature
of Maxim max8997 by using Extcon framework.

The extcon-max8997 driver is implemented based on 'drivers/misc/
max8997-muic.c' and then use Extcon interface instead of callback
function in struct max8997_muic_platform_data to notify cable state
of notifee which want to know always newly cable state when external
connector(e.g., USB, TA, JIG) is attached or detached.

v1
- Use Extcon interface to notify cable state of notifee instead of
 callback function when external connector is attached or detached.
- Bug fix of getting platform_data for irq_base value.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>