AC97 bus register read/write hooks need to provide locking, but the
mpc5200-psc-ac97 driver does not.  This patch adds a mutex around
the register access routines.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>
Acked-by: NJon Smirl <jonsmirl@gmail.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>