powerpc: Fix time code for 601 processors

The 601 doesn't have the timebase register; instead it has an RTCL
register that counts nanoseconds and wraps at 1000000000, and an
RTCU register that counts seconds.  This makes the necessary changes
for the merged time code to use the RTCL/U registers when the kernel
is running on a 601.

Signed-off-by: Paul Mackerras <paulus@samba.org>