alpha: Primitive support for CPU power down.

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: Richard Henderson <rth@twiddle.net>

arch/alpha/kernel/process.c

@@ -46,6 +46,23 @@
 void (*pm_power_off)(void) = machine_power_off;
 EXPORT_SYMBOL(pm_power_off);
 
+#ifdef CONFIG_ALPHA_WTINT
+/*
+ * Sleep the CPU.
+ * EV6, LCA45 and QEMU know how to power down, skipping N timer interrupts.
+ */
+void arch_cpu_idle(void)
+{
+	wtint(0);
+	local_irq_enable();
+}
+
+void arch_cpu_idle_dead(void)
+{
+	wtint(INT_MAX);
+}
+#endif /* ALPHA_WTINT */
+
 struct halt_info {
 	int mode;
 	char *restart_cmd;