Increase BL2U stack size

BL2U is running out of stack during firmware update. Increase
stack size to prevent this

Change-Id: I9b1a4e237a00172c6738c84b455b3878ab184cb8
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>

include/plat/arm/board/common/board_arm_def.h

@@ -28,7 +28,7 @@
 #  define PLATFORM_STACK_SIZE 0x400
 # endif
 #elif defined(IMAGE_BL2U)
-# define PLATFORM_STACK_SIZE 0x200
+# define PLATFORM_STACK_SIZE 0x400
 #elif defined(IMAGE_BL31)
 #if ENABLE_SPM
 # define PLATFORM_STACK_SIZE 0x500