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>