vexpress64: Juno: Declare all 8GB of RAM and make them visible to the kernel.

Juno comes with 8GB RAM, but U-Boot only passes 2GB to the kernel.
Declare a secondary memory bank and set the sizes correctly.

Signed-off-by: Liviu Dudau <Liviu.Dudau@foss.arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>