extcon: intel-cht-wc: Enable external charger

In some configuration external charger "#charge enable" signal is
connected to PMIC. Enable it at device probing to allow charging.

Save CHGRCTRL0 and CHGDISCTR registers at driver probing and restore
them at driver unbind to re-enable hardware charging control if it was
enabled before.

Tested at Lenovo Yoga Book (YB1-X91L).

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>