Move all input device related initialization into a new
axp20x_pek_probe_input_device helper function.

This introduces one functional change, the input device is now
registered before the sysfs attr get registered. This is not a problem
as the sysfs attr are to configure some long press settings (forced
poweroff) in the hardware and do not interact with the input_device.

This is a preparation patch for not always registering the input dev.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>