target/ppc: Disable software TLB for the 7450 family

target/ppc: Disable software TLB for the 7450 family

(Applies to 7441, 7445, 7450, 7451, 7455, 7457, 7447 and 7447a)*

We have since 2011 [1] been unable to run OpenBIOS in the 7450s and
have not heard of any other software that is used with those CPUs in
QEMU. A current discussion [2] shows that the 7450 software TLB is
unsupported in Linux 5.15, FreeBSD 13, MacOS9, MacOSX and MorphOS
3.15. With no known support in firmware or OS, this means that no code
for any of the 7450 CPUs is ever ran in QEMU.

Since the implementation in QEMU of the 7400 MMU is the same as the
7450, except for the software TLB vs. hardware TLB search, this patch
changes all 7450 cpus to the 7400 MMU model. This has the practical
effect of disabling the software TLB feature while keeping other
aspects of address translation working as expected.

This allow us to run software on the 7450 family again.

*- note that the 7448 is currently aliased in QEMU for a 7400, so it
   is unaffected by this change.

1- https://bugs.launchpad.net/qemu/+bug/812398
   https://gitlab.com/qemu-project/qemu/-/issues/86

2- https://lists.nongnu.org/archive/html/qemu-ppc/2021-11/msg00289.html
   message id: 20211119134431.406753-1-farosas@linux.ibm.com

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20211130230123.781844-2-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>