arm: mvebu: Fix compile for multiplatform when ARMv6 is selected

Some systems compile in both ARMv6 and ARMv7 into multiplatform
configurations. This means the default compiler flags are for ARMv6,
and we will get:

arch/arm/mach-mvebu/coherency_ll.S: Assembler messages:
arch/arm/mach-mvebu/coherency_ll.S:45: Error: selected processor does not support `dsb'

Fix this by specifying ARMv7 flags for coherency_ll.o.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>