sgm775/module: Add DDR PHY module

This patch adds a platform-specific DDR PHY module derived
from the generic DDR_PHY500 module providing memory-cost saving
and simplified configuration.

Change-Id: I6c1648ec156401caca4cdb649762a3a57534a95c
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>