ARM: Factor out common psci_get_cpu_id

ARM: Factor out common psci_get_cpu_id

Will be required for obtaining the ID of the current CPU in shared PSCI
functions. The default implementation requires a dense ID space and only
supports a single cluster. Therefore, the functions can be overloaded in
cases where these assumptions do not hold.

CC: Marc Zyngier <[email protected]>
Signed-off-by: Jan Kiszka <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Thierry Reding <[email protected]>
Tested-by: Thierry Reding <[email protected]>
Tested-by: Ian Campbell <[email protected]>
Acked-by: Ian Campbell <[email protected]>
Signed-off-by: Tom Warren <[email protected]>