vexpress64: compile Juno PCIe conditionally

Only compile in PCIe support if the board really uses it. Provide
a __weak stub for the init function if e.g. FVP is being built.

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