copyright: remove IBM copyright

According to https://wiki.linaro.org/Copyright, all work done by assignees is
copyrighted to Linaro.
diff --git a/powerdebug.c b/powerdebug.c
index 871b947..3f4d60c 100644
--- a/powerdebug.c
+++ b/powerdebug.c
@@ -1,6 +1,5 @@
 /*******************************************************************************
- * Copyright (C) 2010, Linaro
- * Copyright (C) 2010, IBM Corporation
+ * Copyright (C) 2010, Linaro Limited.
  *
  * This file is part of PowerDebug.
  *