platform/rdfremontcfg2: add mapping for second dram block

RD-Fremont-Cfg2 is a multichip platform and it supports two blocks of
DRAM memory with 2 GiB on the first block and 6GiB on the second block.
FVP allows the flexibility to configure the base and end address of the
second block of DRAM memory for each chip. So add the DRAM mapping for
this second DRAM block for all chips.

Change-Id: Iaf2e06061303a0c047f7d3f371c80ac540d0b10c
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>