powerpc: Silence software timebase sync

When no hardware method is provided to sync the timebase registers
across the machine, and the platform doesn't sync them for us, then we
use a generic software implementation.  Currently, the code for that
has many printks, and they don't have log levels.  Most of the printks
are only useful for debugging the code, and since we haven't had any
problems with it for years, this turns them into pr_debug.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>