[PATCH] ppc64: Make idle_loop a ppc_md function

This patch adds an idle member to the ppc_md structure and calls it from
cpu_idle().  If a platform leaves ppc_md.idle as null it will get the default
idle loop default_idle().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>