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>

arch/arm/mach-mvebu/system-controller.c

@@ -127,7 +127,7 @@ int mvebu_system_controller_get_soc_id(u32 *dev, u32 *rev)
 }
 
 #if defined(CONFIG_SMP) && defined(CONFIG_MACH_MVEBU_V7)
-void mvebu_armada375_smp_wa_init(void)
+static void mvebu_armada375_smp_wa_init(void)
 {
 	u32 dev, rev;
 	phys_addr_t resume_addr_reg;