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_afflvl_on.c b/services/std_svc/psci/psci_afflvl_on.c
index e3a1831..443e6af 100644
--- a/services/std_svc/psci/psci_afflvl_on.c
+++ b/services/std_svc/psci/psci_afflvl_on.c
@@ -378,7 +378,7 @@
 	 * structure. The calling cpu should have set the
 	 * context already
 	 */
-	assert(cm_get_context(mpidr, NON_SECURE));
+	assert(cm_get_context(NON_SECURE));
 	cm_set_next_eret_context(NON_SECURE);
 	cm_init_pcpu_ptr_cache();
 	write_vbar_el3((uint64_t) runtime_exceptions);