esoc: Perform warm reset in place of cold reset for Kona

As cold reset is not supported on Kona, perform warm reset
internally in the mdm_power_down function. Revert back to using
power down api in reboot and shutdown functions.

Change-Id: I20a615874bfb160889cd74fbfa2c814cada1f4d6
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>