The default DRAM2 start address for Arm platforms
is 0x880000000. However, for N1SDP platform this is
0x8080000000.

Fix the DRAM2 start address by initialising
PLAT_ARM_DRAM2_BASE.

Without this fix there is a mismatch of the System
memory region view as seen by the BL31 runtime
firmware (PSCI) versus the view of the OS (which
is based on the description provided by UEFI. In
this case UEFI is correctly describing the DRAM2
start address).

This implicates in secondary cores failing to start
on some Operating Systems if the OS decides to place
the secondary start address in the mismatched region.

Change-Id: I57220e753219353dda429868b4c5e1a69944cc64
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>