axp152: Add support for power button

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
sunxi: Fix reboot not working on sun5i

Replace our current watchdog based system reset code with code from the
allwinner sun7i 3.3 sources. This fixes reboot hanging once every 2 attempts
on sun5i (tested on A13 and various A10s based boards). Also tested on A10
and A20 based boards to avoid causing a reboot regression there.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>