ftrace, POWERPC: add irqs_disabled_flags to ppc

PPC doesn't have the irqs_disabled_flags needed by ftrace.
This patch adds it.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>