vexpress64: support the Juno Development Platform

The Juno Development Platform is a physical Versatile Express
device with some differences from the emulated semihosting
models. The main difference is that the system is split in
a SoC and an FPGA where the SoC hosts the serial ports at
totally different adresses.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>