mvebu: make mvebu_armada375_smp_wa_init() static

The mvebu_armada375_smp_wa_init() is not exported or declared
anywhere, so make it static to fix the following warning:

arch/arm/mach-mvebu/system-controller.c:130:6: warning: symbol 'mvebu_armada375_smp_wa_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>