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/include/plat/marvell/a8k/common/armada_common.h b/include/plat/marvell/a8k/common/armada_common.h
index dd2a24a..709d009 100644
--- a/include/plat/marvell/a8k/common/armada_common.h
+++ b/include/plat/marvell/a8k/common/armada_common.h
@@ -124,5 +124,6 @@
 			       uint32_t *size, uintptr_t base);
 int marvell_get_ccu_memory_map(int ap_idx, struct addr_map_win **win,
 			       uint32_t *size);
+int system_power_off(void);
 
 #endif /* ARMADA_COMMON_H */