esoc: power down on reboot

Power down external modem during primary reboot. This ensures the
external modem is in a consistent state on the subsequent powerup
after the reboot.

Change-Id: I52da95e450a58f4180153d5e069bd2dae8d6132e
CRs-Fixed: 2244359
Signed-off-by: Adam Bickett <abickett@codeaurora.org>