When adding CPU to the SMP group and enabling the coherency on this
CPU we must protect the register access.
The previous implementation claims to be atomic but doesn't provide
any protection against parallel access to the coherency fabric control
and configuration registers.

This patch fixes this by using the ldrex and strex mechanism.
This method should be used in all accesses to those registers.

[gregory.clement@free-electrons.com: fixed the commit's topic]
Signed-off-by: Nadav Haklai <nadavh@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>