Dual-license IBM code contributions

It was brought to our attention that U-Boot contains code derived from the
IBM OpenBIOS source code originally provided with some of the older PowerPC
4xx development boards.  As a result, the original license of this code has
been carried in the various files for a number of years in the U-Boot project.

IBM is dual-licensing the IBM code contributions already present in U-Boot
under either the terms of the GNU General Public License version 2, or the
original code license already present.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>