power: reset: initialize hard reset code

There is a chance that the system can crash when the device is rebooting
after a warm reset. So mark it as hard reset and store the reset reason
into PON spare register.

Change-Id: I6abb9fa8ed4639f51e929b0126cfb834f97b06b0
Signed-off-by: Lijuan Gao <lijuang@codeaurora.org>