powercap/drivers/dtpm: Save cpuhp dynamic state

Requesting a CPUHP_AP_ONLINE_DYN cpuhp state reserves a state
among the range [CPUHP_AP_ONLINE_DYN:CPUHP_AP_ONLINE_DYN_END].
The reserved state must be saved in order to remove it later.

Cf. __cpuhp_setup_state_cpuslocked() documentation:
Return:
  On success:
    Positive state number if @state is CPUHP_AP_ONLINE_DYN;
    0 for all other states

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>