docs: Add SDEI dispatcher documentation

The document includes SDEI sequence diagrams that are generated using
PlantUML [1].

A shell script is introduced to generate SVG files from PlantUML files
supplied in arguments.

[1] http://plantuml.com/PlantUML_Language_Reference_Guide.pdf

Change-Id: I433897856810bf1927f2800a7b2b1d81827c69b2
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
6 files changed