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>

arch/powerpc/include/asm/machdep.h

@@ -240,6 +240,12 @@ struct machdep_calls {
 	 * claims to support kexec.
 	 */
 	int (*machine_kexec_prepare)(struct kimage *image);
+
+	/* Called to perform the _real_ kexec.
+	 * Do NOT allocate memory or fail here. We are past the point of
+	 * no return.
+	 */
+	void (*machine_kexec)(struct kimage *image);
 #endif /* CONFIG_KEXEC */
 
 #ifdef CONFIG_SUSPEND

arch/powerpc/kernel/machine_kexec.c

@@ -87,7 +87,10 @@ void machine_kexec(struct kimage *image)
 
 	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);