a8k: Implement platform specific power off

Implements a way to add platform specific power off code to a
Marvell Armada 8K platform.

Marvell Armada 8K boards can now add a board/system_power.c file
that contains a system_power_off() function.
This function can now send a command to a power management MCU or
other board periferals before shutting the board down.

Signed-off-by: Luka Kovacic <luka.kovacic@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Change-Id: Iaba20bc2f603195679c54ad12c0c18962dd8e3db
---
I am working on a device that will be ported later, which has a
custom power management MCU that handles LEDs, board power and fans
and requires this separation.
diff --git a/plat/marvell/a8k/common/plat_pm.c b/plat/marvell/a8k/common/plat_pm.c
index d07601a..96e95c2 100644
--- a/plat/marvell/a8k/common/plat_pm.c
+++ b/plat/marvell/a8k/common/plat_pm.c
@@ -792,8 +792,20 @@
  * A8K handlers to shutdown/reboot the system
  *****************************************************************************
  */
+
+/* Set a weak stub for platforms that don't configure system power off */
+#pragma weak system_power_off
+int system_power_off(void)
+{
+	return 0;
+}
+
 static void __dead2 a8k_system_off(void)
 {
+	/* Call the platform specific system power off function */
+	system_power_off();
+
+	/* board doesn't have a system off implementation */
 	ERROR("%s:  needs to be implemented\n", __func__);
 	panic();
 }