board: sl28: add ATF support (bl31)

Add support to load the bl31 part of the ARM Trusted Firmware by the
SPL.

Signed-off-by: Michael Walle <michael@walle.cc>
diff --git a/board/kontron/sl28/Kconfig b/board/kontron/sl28/Kconfig
index cdec39b..aba49fc 100644
--- a/board/kontron/sl28/Kconfig
+++ b/board/kontron/sl28/Kconfig
@@ -15,4 +15,14 @@
 config SYS_TEXT_BASE
 	default 0x96000000
 
+config SL28_SPL_LOADS_ATF_BL31
+	bool "SPL loads BL31 of the ARM Trusted Firmware"
+	select SPL_ATF
+	select SPL_ATF_LOAD_IMAGE_V2
+	select ARMV8_SEC_FIRMWARE_SUPPORT
+	select SEC_FIRMWARE_ARMV8_PSCI
+	help
+	  Enable this to load a BL31 image by the SPL. You have to
+	  provde a bl31.bin in u-boot's root directory.
+
 endif