Marvell/Armada7k8k: Extend ArmadaSoCDescLib with MDIO information

This patch introduces new library callback (ArmadaSoCDescMdioGet ()),
which dynamically allocates and fills MV_SOC_MDIO_DESC structure with
the SoC description of Mdio controllers.

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