test(ff-a v1.2): ffa_run to zero extended registers

FF-A v1.2 introduces an extended set of available registers
for its interfaces. When a partition that is handling a
direct request yields, it may be resumed by ffa_run. In order
to maintain the full extended register state when passing execution
back to the yielding partition, allow ffa_run to zero the extended
register set to preseve runtime state of the calling partition.

Patch also includes tests demonstrating this use case.

Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Change-Id: I27a01ab40f9f74867536812c24a0d665e782754f
diff --git a/src/api.c b/src/api.c
index c76bf85..0e97c09 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1086,7 +1086,7 @@
  * Helper to check if extended arguments (corresponding to regs x8-x17)
  * are zeroed out.
  */
-static bool api_extended_args_are_zero(struct ffa_value *args)
+bool api_extended_args_are_zero(struct ffa_value *args)
 {
 	if (args->extended_val.arg8 != 0U || args->extended_val.arg9 != 0U ||
 	    args->extended_val.arg10 != 0U || args->extended_val.arg11 != 0U ||