PM: EM: Add argument to get_cost() for runtime modification

The Energy Model (EM) supports runtime modifications. Let also the
artificial EM use this new feature and allow to update the 'cost' values
at runtime. When the artificial EM is used there is a need to provide
two callbacks: get_cost() and update_power(), not only the last one.

Update also CPPC driver code, since the new argument is needed there
to compile properly and register EM.

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