[PATCH] powerpc: Allow for ppc_md restart, power_off, and halt to be NULL

On a number of embedded reference boards there isn't always a
way to reset, power_off, or halt the board.  Rather than having
each board implement a spin loop just let the generic code do
it.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/setup-common.c

@@ -100,6 +100,7 @@ void machine_shutdown(void)
 void machine_restart(char *cmd)
 {
 	machine_shutdown();
+	if (ppc_md.restart)
 		ppc_md.restart(cmd);
 #ifdef CONFIG_SMP
 	smp_send_stop();
@@ -112,6 +113,7 @@ void machine_restart(char *cmd)
 void machine_power_off(void)
 {
 	machine_shutdown();
+	if (ppc_md.power_off)
 		ppc_md.power_off();
 #ifdef CONFIG_SMP
 	smp_send_stop();
@@ -129,6 +131,7 @@ EXPORT_SYMBOL_GPL(pm_power_off);
 void machine_halt(void)
 {
 	machine_shutdown();
+	if (ppc_md.halt)
 		ppc_md.halt();
 #ifdef CONFIG_SMP
 	smp_send_stop();