MIPS: Lemote 2F: Ensure atomic execution of _rdmsr and _wrmsr

On Lemote 2F CS5536 MSRs are accessed through a index / data register pair.
The access sequence must be protected by a spinlock to be atomic.

Without this rebooting in fs2f_reboot() may fail.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1058/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>