Docs: dt: add generic MSI bindings

Currently msi-parent is used in a couple of drivers despite being fairly
underspecified. This patch adds a generic binding for MSIs (including
the existing msi-parent property) enabling the description of platform
devices capable of using MSIs.

While MSIs are primarily distinguished by doorbell and payload, some MSI
controllers (e.g. the GICv3 ITS) also use side-band information
accompanying the write to identify the master which originated the MSI,
to allow for sandboxing. This sideband information is non-probeable and
needs to be described in the DT. Other MSI controllers may have
additional configuration details which need to be described per-master.

This patch adds a generic msi-parent binding document, extending the
de-facto standard with a new (optional) #msi-cells which can be used to
express any per-master configuration and/or sideband data. This is
sufficient to describe non-hotpluggable devices.

For busses where sideband data may be derived from some bus-specific
master ID scheme, other properties will be required to describe the
mapping.

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