PM: EM: Force device drivers to provide power in uW

The EM only supports power in uW. Make sure that it is not possible to
register some downstream driver which doesn't provide power in uW.
The only exception is artificial EM, but that EM is ignored by the rest of
kernel frameworks (thermal, powercap, etc).

Reported-by: PoShao Chen <poshao.chen@mediatek.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>