Add option for defining platform DRAM2 base

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>