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
diff --git a/services/std_svc/psci/psci_setup.c b/services/std_svc/psci/psci_setup.c
index a1587b7..015beab 100644
--- a/services/std_svc/psci/psci_setup.c
+++ b/services/std_svc/psci/psci_setup.c
@@ -210,9 +210,9 @@
 		linear_id = platform_get_core_pos(mpidr);
 		assert(linear_id < PLATFORM_CORE_COUNT);
 
-		cm_set_context(mpidr,
-				(void *) &psci_ns_context[linear_id],
-				NON_SECURE);
+		cm_set_context_by_mpidr(mpidr,
+					(void *) &psci_ns_context[linear_id],
+					NON_SECURE);
 
 	}