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The base address for second DRAM varies across different platforms.
So allow platforms to define second DRAM by moving Juno/SGM-775 specific
definition of second DRAM base address to Juno/SGM-775 board definition
respectively, SGI/RD specific definition of DRAM 2 base address to SGI
board definition.

Change-Id: I0ecd3a2bd600b6c7019c7f06f8c452952bd07cae
Signed-off-by: Suyash Pathak <suyash.pathak@arm.com>