hw/xscom: Enable sw xstop by default on p9
hw/xscom: Enable sw xstop by default on p9

This was disabled at some point during bringup to make life easier for
the lab folks trying to debug NVLink issues. This hack really should
have never made it out into the wild though, so we now have the
following situation occuring in the field:

 1) A bad happens
 2) The host kernel recieves an unrecoverable HMI and calls into OPAL to
    request a platform reboot.
 3) OPAL rejects the reboot attempt and returns to the kernel with
    OPAL_PARAMETER.
 4) Kernel panics and attempts to kexec into a kdump kernel.

A side effect of the HMI seems to be CPUs becoming stuck which results
in the initialisation of the kdump kernel taking a extremely long time
(6+ hours). It's also been observed that after performing a dump the
kdump kernel then crashes itself because OPAL has ended up in a bad
state as a side effect of the HMI.

All up, it's not very good so re-enable the software checkstop by
default. If people still want to turn it off they can using the nvram
override.

Cc: 74fabe089ecbd0f3f71683991d9e2780a4de1f4a@lists.ozlabs.org
Cc: Mahesh Salgaonkar <6a890e1065a1d4e8df9dcf1d9a2c9c740bbcf8aa@linux.vnet.ibm.com>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>
Acked-by: Mahesh Salgaonkar <6a890e1065a1d4e8df9dcf1d9a2c9c740bbcf8aa@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

stewart-ibm/skiboot,shenki/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,stewart-ibm/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot