refactor(spm): helper functions for SPM's MP tests

Factored out code to power on cpus from direct messaging test, and
placed in a helper function. To be used to test other multicore
functionality.
The function expects the cpu on handler to be implemented in the scope
of the functionality to be tested.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Ic0074dc85e0a906bae9d7b8cc071aff476f94de5
3 files changed