Fix Foundation FVP instructions in User Guide

The Arm Trusted Firmware is built by default for ARMv8-A version 8.0.
However, the Foundation FVP runs by default in the highest version of
the architecture it supports. This causes problems when trying to run
the Arm Trusted Firmware on it.

This patch adds a note to the User Guide about this problem.

Change-Id: I0220fe1a9c66c2292149ad4a7ffe5e27ba08ab28
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>