ftrace, powerpc: replace debug macro with proper pr_deug

Impact: cleanup

The PowerPC ftrace code uses a hacked up DEBUGP macro for prints.
This patch converts it to the standard pr_debug.

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