doc: Document more platforms
doc: Document more platforms

These platforms are supported in the tree but didn't make it to the docs
folder yet.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

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