SPDX: fix IBM-pibs license identifier

The SPDX License List version 1.19 now contains an official entry for
the IBM-pibs license.  However, instead of our suggestion "ibm-pibs",
the SPDX License List uses "IBM-pibs", with the following rationale:
"The reason being that all other SPDX License List short identifiers
tend towards using capital letters unless spelling a word.  I'd prefer
to be consistent to this end".

Change the license IDs to use the official name.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/arch/powerpc/include/asm/ppc440.h b/arch/powerpc/include/asm/ppc440.h
index 0f5bc8d..0cfa88b 100644
--- a/arch/powerpc/include/asm/ppc440.h
+++ b/arch/powerpc/include/asm/ppc440.h
@@ -9,7 +9,7 @@
  * (C) Copyright 2010
  * Stefan Roese, DENX Software Engineering, sr@denx.de.
  *
- * SPDX-License-Identifier:	GPL-2.0	ibm-pibs
+ * SPDX-License-Identifier:	GPL-2.0	IBM-pibs
  */
 
 #ifndef __PPC440_H__