The default DRAM2 base address for Arm platforms
is 0x880000000. However, on some platforms the
firmware may want to move the start address to
a different value.

To support this introduce PLAT_ARM_DRAM2_BASE that
defaults to 0x880000000; but can be overridden by
a platform (e.g. in platform_def.h).

Change-Id: I0d81195e06070bc98f376444b48ada2db1666e28
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>