configs: add support for configuring the DRAM size

The macro PLAT_DRAM2_SIZE is used to configure the total memory size of
the platform. The Platform's memory space is physically divided into two
blocks. The default total memory size is 8GB, and the first block size
is 2GB. Therefore, the default value of the second block,
PLAT_DRAM2_SIZE, is calculated as 6GB. The macro PLAT_DRAM2_SIZE is
passed as a build flag and is used in both TF-A and UEFI.

This support is only available for rdv2 and variants of rdn2 platforms.
Any changes made to the PLAT_DRAM2_SIZE value will require a
corresponding change in the model-scripts.

Signed-off-by: Tony Nadackal <tony.nadackal@arm.com>
Change-Id: Iafd7045e87900aa0eb949840ddf685bd6d7a6840