powerpc: enable dynamic ftrace

This patch enables dynamic ftrace. The PowerPC port was dependent on
other code not yet in mainline. Now that the code is, we can now
let PowerPC compile with dynamic ftrace.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>