marvell: a8k: set BL33 to /dev/null

TF-A build for Marvell a8k requires u-boot binary to be specified in
BL33 variable. As CI does not test and check compilation of u-boot, set
BL33 just to /dev/null like it is for SCP_BL2.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: If3bc38a0dcd9891a6bd5ae4ff17a879e43a0f0b8
diff --git a/tf_config/a80x0-default b/tf_config/a80x0-default
index be2c5d8..ee6617d 100644
--- a/tf_config/a80x0-default
+++ b/tf_config/a80x0-default
@@ -1,3 +1,4 @@
 CROSS_COMPILE=/arm/pdsw/tools/gcc-linaro-6.2.1-2016.11-x86_64_aarch64-linux-gnu/bin/aarch64-linux-gnu-
 PLAT=a80x0
 SCP_BL2=/dev/null
+BL33=/dev/null