vexpress64: remove board late init, use smhload

This removes the kludgy late board init from the FVP simulator
version of Versatile Express 64bit (ARMv8), and replace it with
a default boot command using the new smhload command to load
the files using semihosting. Tested on the Foundation Model.

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