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
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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.