PM: EM: Add RCU mechanism which safely cleans the old data

The EM is going to support runtime modifications of the power data.
In order to achieve that prepare the internal mechanism. This patch
introduces RCU safe mechanism to clean up the old allocated WM data.
It also adds a mutex for the EM structure to serialize the modifiers.
The writers which will block on that mutex must be running the sleeping
context.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>