doc/device-tree: Add descriptions for the ibm, pstate-* properties.
doc/device-tree: Add descriptions for the ibm, pstate-* properties.

Signed-off-by: Jeremy Kerr <68a658b44bbaefab3ef5cd231967195fc0ff9d69@ozlabs.org>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

apopple/skiboot,mikey/skiboot,stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,mikey/skiboot,open-power/skiboot,stewart-ibm/skiboot,legoater/skiboot,apopple/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,cyrilbur-ibm/skiboot,jk-ozlabs/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,apopple/skiboot,legoater/skiboot,csmart/skiboot,jk-ozlabs/skiboot,shenki/skiboot,shenki/skiboot,csmart/skiboot,qemu/skiboot,ddstreet/skiboot,stewart-ibm/skiboot,shenki/skiboot,ddstreet/skiboot,qemu/skiboot,mikey/skiboot,csmart/skiboot,ddstreet/skiboot,shenki/skiboot,jk-ozlabs/skiboot,cyrilbur-ibm/skiboot,cyrilbur-ibm/skiboot,qemu/skiboot