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/board/mpl/mip405/init.S b/board/mpl/mip405/init.S
index bf886c0..642f17c 100644
--- a/board/mpl/mip405/init.S
+++ b/board/mpl/mip405/init.S
@@ -1,5 +1,5 @@
 /*
- * SPDX-License-Identifier:	GPL-2.0	ibm-pibs
+ * SPDX-License-Identifier:	GPL-2.0	IBM-pibs
  */
 /*-----------------------------------------------------------------------------
  * Function:     ext_bus_cntlr_init