x86, microcode, AMD: Add a vendor-specific exit function

This will be used to do cleanup work before the driver exits.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>

arch/x86/kernel/microcode_amd.c

@@ -353,3 +353,7 @@ struct microcode_ops * __init init_amd_microcode(void)
 {
 	return &microcode_amd_ops;
 }
+
+void __exit exit_amd_microcode(void)
+{
+}