Docs: dt: Add PCI MSI map bindings

Currently msi-parent is used by a few bindings to describe the
relationship between a PCI root complex and a single MSI controller, but
this property does not have a generic binding document.

Additionally, msi-parent is insufficient to describe more complex
relationships between MSI controllers and devices under a root complex,
where devices may be able to target multiple MSI controllers, or where
MSI controllers use (non-probeable) sideband information to distinguish
devices.

This patch adds a generic binding for mapping PCI devices to MSI
controllers. This document covers msi-parent, and a new msi-map property
(specific to PCI*) which may be used to map devices (identified by their
Requester ID) to sideband data for each MSI controller that they may
target.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>