powerpc/64s: Move IDLE_STATE_ENTER_SEQ[_NORET] into idle_book3s.S

This macro is only used in idle_book3s.S, move it in there and add a
more descriptive comment.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out of larger patch and write change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>