vexpress64: fvp dram: add DRAM configuration

Create an additional FVP configuration to boot images pre-loaded into
DRAM.

Sometimes it's preferential to boot the model by loading the files
directly into DRAM via model parameters, rather than using
SemiHosting.

An example of model parmaters that are used to pre-load the files
into DRAM:
    --data cluster0.cpu0=Image@0x80080000 \
    --data cluster0.cpu0=fvp-base-gicv2-psci.dtb@0x83000000 \
    --data cluster0.cpu0=uInitrd@0x84000000

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
[trini: Update board/armltd/vexpress64/Kconfig logic]
Signed-off-by: Tom Rini <trini@konsulko.com>