ppc: e6500 registers SPR 604 twice

ppc: e6500 registers SPR 604 twice

When using the e6500 CPU, QEMU generates a fatal error after
complaining about registering SPR 604 twice.

Building and testing with commit
9b2e891ec5ccdb4a7d583b77988848282606fdea shows the issue:

qemu-system-ppc64 --version
QEMU emulator version 3.1.50 (v3.1.0-456-g9b2e891ec5-dirty)
Copyright (c) 2003-2018 Fabrice Bellard and the QEMU Project developers

qemu-system-ppc64 -M none -cpu e6500
Error: Trying to register SPR 604 (25c) twice !

Signed-off-by: Jon Diekema <jon.diekema@ge.com>
Message-Id: <CALvuzg43uSodseEHjNaRcPFBKKPTY2mcppUbYgiLL=QO9RxX_Q@mail.gmail.com>
[removed duplicated mail header in the commit message]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>