platform/mihawk: Detect old system compatible string
platform/mihawk: Detect old system compatible string

Newer firmware declares the system as "ibm,mihawk", but the labs are
full of older installs, which were using "wistron,mihawk". Let's keep
detecting the older string since it allows to run recent skiboot on
older fw stack and make people's lives a little tiny bit easier.

Reviewed-by: Andrew Donnellan <44ca8c041ed925606732b2a0f30e5b73c32761a8@linux.ibm.com>
Reviewed-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@flamingspork.com>
Signed-off-by: Frederic Barrat <c90240987c583ac0802fbeb88cb1684edc683e18@linux.ibm.com>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

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