Input: add driver for AXP20x Power Enable Key

This change adds support for the Power Enable Key found on MFD AXP202
and AXP209. Besides the basic support for the button, the driver adds
two entries in sysfs to configure the time delay for power on/off.

Signed-off-by: Carlo Caione <carlo@caione.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
[wens@csie.org: made axp20x_pek_remove() static; removed driver owner
 field; fixed path for sysfs entries]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>