mvebu: add definition for coherency_base

Fix the warning that coherency_base is not defined by adding
it to coherency.h (it is only used in the coherency_ll.S):

arch/arm/mach-mvebu/coherency.c:41:14: warning: symbol 'coherency_base' 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/coherency.h

@@ -14,6 +14,7 @@
 #ifndef __MACH_370_XP_COHERENCY_H
 #define __MACH_370_XP_COHERENCY_H
 
+extern void __iomem *coherency_base;	/* for coherency_ll.S */
 extern unsigned long coherency_phys_base;
 int set_cpu_coherent(void);