The Qualcomm Snapdragon 410 is Qualcomm's first 64-bit SoC, released
in 2014 with four ARM Cortex-A53 cores. There are differents variants
(MSM8916, APQ8016(E), ...) that are all very similar. A popular device
based on APQ8016E is the DragonBoard 410c single-board computer,
but the SoC is also used in various mid-range smartphones/tablets.

This commit adds documentation for a minimal, community-maintained port
of TF-A/BL31 for MSM8916. The actual platform port is added in the
following four separate small commits to simplify the review process.
The code is primarily based on the information from the public
Snapdragon 410E Technical Reference Manual [1], combined with a lot of
trial and error to actually make it work.

Note that this port is a pure community effort without any
commercial interests and is not related to Qualcomm in any way.
The main motivation for this port is to have a minimal, updatable
firmware since this old chip does not receive many updates anymore from
Qualcomm. It works quite well for many use cases so I am willing to
maintain it as a "code owner". I have also added Nikita Travkin as
second code owner to help with reviews.

The main limitation so far is the lack of memory protection for TF-A.
This is similar to the ports for the Raspberry Pi but in this case not
a lack of hardware support but rather a lack of documentation. However,
this does not limit the usefulness of the port when used as a minimal
PSCI implementation.

[1]: https://developer.qualcomm.com/download/sd410/snapdragon-410e-technical-reference-manual.pdf



Change-Id: I676adf86061638cfc2f3ae8615470d145e84f172
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>