powerpc/kexec: Restore ppc_md.machine_kexec

Kyle Moffett points out that mpc85xx has started using the
ppc_md.machine_kexec hook. As such, revert patch c94868788cf2
(powerpc/kexec: Remove ppc_md.machine_kexec).

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/kernel/machine_kexec.c b/arch/powerpc/kernel/machine_kexec.c
index 49a170a..a5f8672 100644
--- a/arch/powerpc/kernel/machine_kexec.c
+++ b/arch/powerpc/kernel/machine_kexec.c
@@ -87,7 +87,10 @@
 
 	save_ftrace_enabled = __ftrace_enabled_save();
 
-	default_machine_kexec(image);
+	if (ppc_md.machine_kexec)
+		ppc_md.machine_kexec(image);
+	else
+		default_machine_kexec(image);
 
 	__ftrace_enabled_restore(save_ftrace_enabled);