vexpress64: take over maintenance of the semi vexpress64

As agreed with Steve Rae I'm taking over maintenance of the
semihosted, emulated FVP/foundation model Versatile Express
64 bit board variant.

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