Merge pull request #472 from danh-arm/dh/fwu-docs ...

FWU: Add documentation for Firmware Update feature

FWU: Add documentation for Firmware Update feature ...

This patch adds design documentation for the Firmware Update (FWU)
feature in `firmware-update.md`. It provides an overview of FWU,
describes the BL1 SMC interface, and includes diagrams showing
an example FWU boot flow and the FWU state machine.

This patch also updates the existing TF documents where needed:

*   `porting-guide.md`
*   `user-guide.md`
*   `firmware-design.md`
*   `rt-svc-writers-guide.md`
*   `trusted_board_boot.md`

Change-Id: Ie6de31544429b18f01327bd763175e218299a4ce
Co-Authored-By: Dan Handley <dan.handley@arm.com>

Introduce the ARM TF reset design document ...

This patch introduces a new document presenting the ARM Trusted
Firmware Reset Design. It shows the reset code flow, lists the
different build options that affect it, in which case to use them
and what their exact effect is.

The section about using BL31 entrypoint as the reset address has
been moved from the general firmware design document to this one.
It's also been improved to explain why the FVP port supports the
RESET_TO_BL31 configuration, even though the reset vector address
can't be programmed dynamically.

This document includes some images, which have been generated using
Dia version 0.97.2. This tool can be obtained from:
https://wiki.gnome.org/Apps/Dia/Download
This patch provides:
 - the image files describing the different reset flow diagrams;
 - the source '.dia' file;
 - a script automating the generation of the images from the '.dia'
   file.
Note that the 2 latter files are not actually needed for the document
and are provided for convenience only, in case the reset images need
to be modified.

Change-Id: Ib6302e8209d418a5b31c4e85e55fd9e83caf2ca2

PSCI: Add documentation and fix plat_is_my_cpu_primary() ...

This patch adds the necessary documentation updates to porting_guide.md
for the changes in the platform interface mandated as a result of the new
PSCI Topology and power state management frameworks. It also adds a
new document `platform-migration-guide.md` to aid the migration of existing
platform ports to the new API.

The patch fixes the implementation and callers of
plat_is_my_cpu_primary() to use w0 as the return parameter as implied by
the function signature rather than x0 which was used previously.

Change-Id: Ic11e73019188c8ba2bd64c47e1729ff5acdcdd5b

Document design of the Interrupt Mangement Framework ...

This patch adds documentation that describes the design of the Interrupt
management framework in the ARM Trusted Firmware. The porting-guide.md has also
been updated to describe the interface that should be implemented by each
platform to support this framework.

Change-Id: I3eda48e5c9456e6a9516956bee16a29e366633b7
Co-Authored-By: Soby Mathew <soby.mathew@arm.com>

Add EL3 runtime services and SPD documentation ...

1. Add design information on EL3 runtime services and
Secure-EL1 Payload Dispatchers (SPD) to
firmware-design.md.

2. Create new EL3 runtime service writer's guide
(rt-svc-writers-guide.md) to ease creation of new
runtime services.

Change-Id: I670aeb5fc246e25c6e599a15139aac886a0074fd