powerpc: Disable HCALL_STATS by default

The overhead of HCALL_STATS is quite high and the functionality is very
rarely used.  Key statistics are also missing (eg min/max).

With the new hcall tracepoints much more powerful tracing can be done in
a kernel module.  Lets disable this by default.

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