Marvell/Armada7k8k: Use common .fdf file

As for the preparation for adding multiple boards support,
move common part of the .fdf file to the SoC family directory
and change output FD file name to more generic.

Once needed, possible per-board differences will be resolved
by including custom .fdf.inc file. This way adding new common
changes for entire SoC family won't require any
duplication and at the same time the per-board .fdf.inc
will allow better suiting the .FD file contents.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>