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>