build(imx8ulp): add build configs for imx8ulp

Add build configs for i.MX8ULP.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: I40ba84037a38034c596f78718d96d0f2551e1432
diff --git a/tf_config/imx8ulp-default b/tf_config/imx8ulp-default
new file mode 100644
index 0000000..f93a82d
--- /dev/null
+++ b/tf_config/imx8ulp-default
@@ -0,0 +1,2 @@
+CROSS_COMPILE=aarch64-none-elf-
+PLAT=imx8ulp