boot/vexpress-firmware: add upstream URL in Config.in help text

Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/vexpress-firmware/Config.in

@@ -4,3 +4,5 @@ config BR2_TARGET_VEXPRESS_FIRMWARE
 	help
 	  Versatile Express firmware from ARM, with Linaro mods last
 	  change.
+
+	  https://git.linaro.org/arm/vexpress-firmware.git