SPDX-License-Identifier: convert PIBS licensed files

This commit adapts the files that were derived from PIBS (PowerPC
Initialization and Boot Software) codeto using SPDX License
Identifiers.

So far, SPDX has not assigned an official License ID for the PIBS
license yet, so this should be considered preliminary.

Note that the following files contained incorrect license information:

arch/powerpc/cpu/ppc4xx/4xx_uart.c
arch/powerpc/cpu/ppc4xx/start.S
arch/powerpc/include/asm/ppc440.h

These files included, in addition to the GPL-2.0 / ibm-pibs dual
license as inherited from PIBS, a GPL-2.0+ license header which was
obviously incorrect. This has been removed.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Wolfgang Denk <wd@denx.de>

Conflicts:
Licenses/README
Acked-by: Stefan Roese <sr@denx.de>