powerpc/64: Make server perfmon only built on ppc64 server devices

The 64-bit Book-E parts (to date) dont utilize the 'server' class
perfmon.  So building or depending on it makes no sense (and does break
FSL Book-E 64-bit support).  Move the selection of PPC_HAVE_PMU_SUPPORT
to be based on PPC_BOOK3S_64.

Based on a patch from Scott Wood.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>