doc: Suggest to use the latest version of GCC 8.2

The latest version of GCC are required to use the new features of TF-A.
Suggest to use the latest version available on developer.arm.com instead
of the version specified on the Linaro Release notes.
At the time of writing, GCC 8.2-2019.01 is the latest version available.

Change-Id: Idd5c00749e39ca9dc8b7c5623b5d64356c9ce6e5
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>