vexpress64: switch to generic board

The few Versatile Express ARMv8 platforms we have may just as
well be switched to generic board from the beginning.

Tested on the ARM foundation model and the in progress support
for the ARMv8 Juno board.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Steve Rae <srae@broadcom.com>