fix(hftest): prevent hftest to emit PSCI calls from SPs

The hftest framework when using uart control implementation emits a
PSCI_SYSTEM_RESET call when a test terminates as a matter of controlling
automation.
Such call is fine from the primary VM but is forbidden for secondary VMs
and SPs. It generates following errors in test logs:
NOTICE: SMC 0x84000009 attempted from VM 0x8001, blocked=1

At tail of standalone_main_secure.c, kmain calls hftest_ctrl_finish,
hftest_device_reboot, arch_reboot, then smc32(PSCI_SYSTEM_RESET).
This change adds hftest ctrl implementation reboot helpers which calls
hftest_device_reboot in the case of VMs. SPs end in an infinite wait
rather than rebooting.

Change-Id: If493f1620b143b33e87d5be1a9694af8140ec019
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
diff --git a/test/hftest/standalone_main.c b/test/hftest/standalone_main.c
index 10e9ef0..5e6a632 100644
--- a/test/hftest/standalone_main.c
+++ b/test/hftest/standalone_main.c
@@ -52,4 +52,5 @@
 
 out:
 	hftest_ctrl_finish();
+	hftest_ctrl_reboot();
 }