vexpress: Pass LOADADDR to Makefile

Due to upstream commit 938f94cd
("ARM: 7656/1: uImage: Error out on build of multiplatform without LOADADDR"),
vexpress BSP need LOADADDR for making uImage, so pass LOADADDR in the Makefile
for vexpress.

Signed-off-by: Yang Shi <yang.shi@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>