arm: mvebu: mark functions of armada-370-xp.c as static

All the functions in armada-370-xp.c are called from the
DT_MACHINE_START function pointers, so there is no need for them to be
visible outside of this file, and we therefore mark them as static.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>