n1sdp: add API in n1sdp_dmc620 to expose total DDR memory size

This patch creates an API in n1sdp_dmc620 module to expose the
total size of DDR memory connected. The information will be
used by n1sdp_system module to pass it on to BL31 & BL33.

Change-Id: I235596929aadca3acf0e43543c67ea031e256ee8
Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>