[MIPS] Save write-only Config.OD from being clobbered

Save the Config.OD bit from being clobbered by coherency_setup(). This
bit, when set, fixes various errata in the early steppings of Au1x00
SOCs.  Unfortunately, the bit was write-only on the most early of them.
In addition, also restore the bit after a wakeup from sleep.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>