vexpress64: juno: add optional initrd

Some OS images require an initrd on Juno.

If the file ramdisk.img exists in NOR flash, then we load it and pass
the address to the kernel.  Otherwise, we pass the "-" parameter as
before.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>