powerpc/85xx: Fix synchronization of timebase on MP boot

powerpc/85xx: Fix synchronization of timebase on MP boot

There is a small ordering issue in the master core in that we need to
make sure the disabling of the timebase in the SoC is visible before we
set the value to 0.  We can simply just read back the value to
synchronizatize the write, before we set TB to 0.

Reported-by: Dan Hettena
Tested-by: Dan Hettena
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>