Use WTINT to wait for the next interrupt.  Squash the WTINT call
if the PALcode doesn't support it (e.g. MILO).  No attempt is yet
made to skip clock ticks during normal scheduling in order to stay
in power down mode longer.
Signed-off-by: NRichard Henderson <rth@twiddle.net>

Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NThomas Gleixner <tglx@linutronix.de>
Acked-by: NMichael Kerrisk <mtk.manpages@gmail.com>
Acked-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NDave Jones <davej@redhat.com>
Acked-by: NMichael Cree <mcree@orcon.net.nz>
Acked-by: NMatt Turner <mattst88@gmail.com>