[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 <[e-mail]>
Signed-off-by: Paul Mackerras <[e-mail]>

arch/powerpc/kernel/setup-common.c

···
 void machine_restart(char *cmd)
 {
 	machine_shutdown();
-	ppc_md.restart(cmd);
+	if (ppc_md.restart)
+		ppc_md.restart(cmd);
 #ifdef CONFIG_SMP
 	smp_send_stop();
 #endif
···
 void machine_power_off(void)
 {
 	machine_shutdown();
-	ppc_md.power_off();
+	if (ppc_md.power_off)
+		ppc_md.power_off();
 #ifdef CONFIG_SMP
 	smp_send_stop();
 #endif
···
 void machine_halt(void)
 {
 	machine_shutdown();
-	ppc_md.halt();
+	if (ppc_md.halt)
+		ppc_md.halt();
 #ifdef CONFIG_SMP
 	smp_send_stop();
 #endif