Provide cm_get/set_context() for current CPU

All callers of `cm_get_context()` pass the calling CPU MPIDR to the
function. Providing a specialised version for the current
CPU results in a reduction in code size and better readability.

The current function has been renamed to `cm_get_context_by_mpidr()`
and the existing name is now used for the current-CPU version.

The same treatment has been done to `cm_set_context()`, although
only both forms are used at present in the PSCI and TSPD code.

Change-Id: I91cb0c2f7bfcb950a045dbd9ff7595751c0c0ffb