plat(sgi): enable support for dram size configuration

Allowing build time configuration of DRAM2 size. If the macro
PLAT_DRAM2_SIZE is defined, during build time it is used to configure
PLAT_ARM_DRAM2_SIZE instead of the default value.

Example make command for using this flag is given below
make -j 24 PLAT=rdn2 ARCH=aarch64 DEBUG=1
     CFLAGS=-DPLAT_DRAM2_SIZE=0x180000000 CSS_SGI_PLATFORM_VARIANT=3 all

Signed-off-by: Tony Nadackal <tony.nadackal@arm.com>
Change-Id: I5b637151841d91ce41c93186310c893128040968