powerpc: update comments for generic idle conversion

As of commit 799fef0 ("powerpc: Use generic idle loop"), this
applies to arch_cpu_idle() instead of cpu_idle().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/powerpc/kernel/irq.c

@@ -304,7 +304,7 @@ void notrace restore_interrupts(void)
  * being re-enabled and generally sanitized the lazy irq state,
  * and in the latter case it will leave with interrupts hard
  * disabled and marked as such, so the local_irq_enable() call
- * in cpu_idle() will properly re-enable everything.
+ * in arch_cpu_idle() will properly re-enable everything.
  */
 bool prep_irq_for_idle(void)
 {