core/pci: Document some stuff
core/pci: Document some stuff

Document the bridge class code hack and what ibm,pci-config-space-type
actually means. Also replace some of the pci_cap() calls with a variable
to make it a bit more readable.

Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

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