Documentation: Add support for TI System Control Interface (TI-SCI) protocol

Texas Instrument's System Control Interface (TI-SCI) Message Protocol
is used in Texas Instrument's System on Chip (SoC) such as those in
newer SoCs in the keystone processor family starting with K2G.

This message protocol is used to communicate between various compute
or processing entities (such as ARM, DSP etc.) with a central system
controller entity.

TI-SCI message protocol provides support for management of various
hardware entities within the SoC.

The message protocol can be found here:
http://processors.wiki.ti.com/index.php/TISCI

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tero Kristo <t-kristo@ti.com>