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>