fix(hftest): define stacks for all secondary cores

The hftest common libraries was reusing the same stack section
for all secondary cores.
This means some tests would fail unexpectidly.

Adding a function to common.c and service_common.c, which have
the same purpose in the target they are used in, to retrieve
the buffers for the secondary core stacks defined globally.

Signed-off-by: J-Alves <joao.alves@arm.com>
Signed-off-by: K-Meakin <karl.meakin@arm.com>
Change-Id: If93aed330a88c3b52e5771d09911296f2c6214ed
diff --git a/test/vmapi/ffa_secure_partitions/secure_interrupts.c b/test/vmapi/ffa_secure_partitions/secure_interrupts.c
index da0e171..0f5e3ce 100644
--- a/test/vmapi/ffa_secure_partitions/secure_interrupts.c
+++ b/test/vmapi/ffa_secure_partitions/secure_interrupts.c
@@ -549,8 +549,9 @@
 		args.vcpu_id = i;
 		HFTEST_LOG("Booting CPU %u - %lx", i, cpu_id);
 
-		EXPECT_EQ(hftest_cpu_start(cpu_id, cpu_entry_sp_sleep_loop,
-					   (uintptr_t)&args),
+		EXPECT_EQ(hftest_cpu_start(
+				  cpu_id, hftest_get_secondary_ec_stack(i),
+				  cpu_entry_sp_sleep_loop, (uintptr_t)&args),
 			  true);
 
 		/* Wait for CPU to release the lock. */