marvell: pm: do not panic by default in cpu_standby

Current default behavior of cpu_standby callback
is problematic during the SBSA test, which is
unable to run due to EL3 panic. Make it dependent on
the PM firmware running.

Change-Id: I7a53de8c880bd23b157dd65ce14bb48b5a5c76c8
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Reviewed-by: Kostya Porotchkin <kostap@marvell.com>

plat/marvell/a8k/common/plat_pm.c

@@ -379,8 +379,10 @@ static int a8k_validate_power_state(unsigned int power_state,
  */
 static void a8k_cpu_standby(plat_local_state_t cpu_state)
 {
+	if (!is_pm_fw_running()) {
 		ERROR("%s: needs to be implemented\n", __func__);
 		panic();
+	}
 }
 
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