[kernel][pdev][power] Add support for kernel power drivers

This allows targets to have custom kernel drivers for power management
(for now, just rebooting and shutting down).

If no power driver is specified in the MDI,
a default PSCI based implementation will be used instead.

ZX-1506 #comment

Change-Id: I589fd0d300567b3640ccf5676b1101190a303fa9