arm: mvebu: don't hardcode a physical address in headsmp.S

Now that the coherency_init() function is called a bit earlier, we can
actually read the physical address of the coherency unit registers
from the Device Tree, and communicate that to the headsmp.S code,
which avoids hardcoding a physical address.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>