SDEI: Update doc to clarify delegation ...

The explicit event dispatch sequence currently depicts handling done in
Secure EL1, although further error handling is typically done inside a
Secure Partition. Clarify the sequence diagram to that effect.

Change-Id: I53deedc6d5ee0706626890067950c2c541a62c78
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>

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>