soc/cavium/cn81xx: Use ATF from blobs repo

Use precompiled BL31 from blobs repo.

There's no check for USE_BLOBS here as the included file is "free":
The BL31 is Open-Source and licensed under BSD.

Change-Id: I7e9eb429d11150d43aa070d1bd6a11ea71951ce3
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/25751
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
diff --git a/src/soc/cavium/cn81xx/Kconfig b/src/soc/cavium/cn81xx/Kconfig
index 3191ef3..edc9480 100644
--- a/src/soc/cavium/cn81xx/Kconfig
+++ b/src/soc/cavium/cn81xx/Kconfig
@@ -17,6 +17,10 @@
 
 if SOC_CAVIUM_CN81XX
 
+config ARM64_BL31_EXTERNAL_FILE
+	string
+	default "3rdparty/blobs/soc/cavium/cn81xx/bl31.elf"
+
 config ARCH_ARMV8_EXTENSION
 	int
 	default 1