platforms/rd: add support for configuring the dram size

Total memory size of the platform can be configred using the model
parameter board.dram_size. The macro PLAT_DRAM_SIZE determines the total
DRAM size and its default value is set at 8GB. Any changes to the value
of this macro require a corresponding change in the platform code.

The above model parameter is supported only in rdv2 and variants of rdn2
platforms.

Signed-off-by: Tony Nadackal <tony.nadackal@arm.com>
Change-Id: I87db0f696ca34e0cb5947a30b91a3a42d7a96771