Add documentation of the xlat tables library V2

Add documentation of the xlat tables library V2

The documentation describes the design of the translation tables
library version 2 used by the ARM Trusted Firmware.

The diagram file has been created with Dia version 0.97.2. This tool
can be obtained from: https://wiki.gnome.org/Apps/Dia/Download

Inkscape has been used to generate the *.png file from the *.dia file
to work around a bug in the generation of *.png files in some versions
of Dia.

Change-Id: Ie67d9998d4ae881b2c060200a318ad3ac2fa5e91
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>