powerpc: Fix major revision number for Freescale cores

Some 74xx cores by Freescale are using the configuration field instead
of the major revision field for their revision number.  This corrects
the wrong behaviour for those ppc cores including my one.

There is a reference document at Freecale.  It describes the PVR
register.  This is based on that pdf.  You can find the document at:

http://www.freescale.com/files/archives/doc/support_info/PPCPVR.pdf

Signed-off-by: Martin Langer <martin-langer@gmx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>