Marvell/Armada7k8k: Introduce support for Armada-8040-Db

Add new board description file Armada80x0Db.dsc,
which uses common Armada7k8k.fdf file.
Most of the interfaces are fully functional, except for:
- USB ports - it requires merging GPIO support and VBUS
  power supply enabling

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>