xive/p9: Clarify indirect table allocation
xive/p9: Clarify indirect table allocation

The XIVE interrupt controller uses a set of Virtualization Structure
Tables (VST) which characteristics, type, address, size, are described
by Virtual Structure Descriptors (VSD). A VSD is 64bit wide.

The EQ and VP tables are indirect tables. The VSD points to a single
page of VSDs each pointing to a page of virtual structures. Indirect
tables are limited to a single top page which is enough to cover the
whole range of EQs (24 bits) and VPs (19bits).

Signed-off-by: Cédric Le Goater <2d8cace2254d4e43578d21933db015d84bcc468a@kaod.org>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

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