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>
diff --git a/board/armltd/vexpress64/Makefile b/board/armltd/vexpress64/Makefile
index a35db40..b4391a7 100644
--- a/board/armltd/vexpress64/Makefile
+++ b/board/armltd/vexpress64/Makefile
@@ -5,4 +5,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	:= vexpress64.o pcie.o
+obj-y	:= vexpress64.o
+obj-$(CONFIG_TARGET_VEXPRESS64_JUNO)	+= pcie.o