powerpc: Minor cleanup to lib/Kconfig.debug

We don't need an explicit PPC64 in the DEBUG_PREEMPT dependancies as all
PPC platforms now support TRACE_IRQFLAGS_SUPPORT.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>