The Marvell Armada 37xx SoCs-based platforms contain a bit
awkward directory structure because the currently only one
supported PLAT and PLAT_FAMILY are the same. Modify the latter
to 'a3k' in order to improve it and keep plat/marvell/armada
tree more consistent:

plat/marvell/
├── armada
│   ├── a3k
│   │   ├── a3700

[...]

│   ├── a8k
│   │   ├── a70x0

[...]

Change-Id: I693a6ef88e6ce49a326a3328875c90bbc186066a
Signed-off-by: Marcin Wojtas <mw@semihalf.com>