vexpress64: fix various memory issues

The ARM Trusted Firmware or other security solutions are
eating memory from the top of the physical SDRAM1 space,
moving backward from 0xffffffff, currently occupying e.g.
0xfe000000-0xffffffff with Trusted Firmware.

This solution to reserving memory for secure world is not
optimal, so we need to think of how the secure world and
earlier boot stages should communicate to U-Boot what
memory they are eating up. For now let's just put 16MB
aside.

Also enable the memory test command and define start and
end of the test range so we can check that we actually have
all that memory available and working.

Suggested-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>