powerpc: update comments for generic idle conversion

As of commit 799fef06

 ("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>