disable warm reboot on witherspoon

The concept of warm reboot (reboot the host without removing chassis
power) was added a while back as a part of clarifying support for
the Redfish ComputerSystem.Reset Action. It has since been found that
some IBM hardware, including Witherspoon, does not support warm reboots.
The issue is intermittent and depends on the reboot being requested
while the host is in the middle of some i2c transactions. If a warm
reboot occurs during this window, then on the subsequent boot the i2c
hardware will be in a bad state and the host firmware is unable to
recover it.

Given this issue, it has been requested that BMC software go back to
doing cold reboots (cycling power to the chassis during any reboot
request) on this hardware.

The issue has only been reported on Witherspoon so for now only disable
it on that platform.

Tested:
- Verified within QEMU that an image with this commit used the cold reboot
  target during a warm reboot request

(From meta-ibm rev: 567a8dcf84dfb74e7bd27d6239be2878fcacdde6)

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I8d77c5e8bf7378a3d1accc79b510f14626a333e5
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>