rockchip: firefly-rk3399: add FIT for rk3399

Enable SPL_FIT_GENERATOR with path for it.
With this patch you can get u-boot.itb for rk3399-firefly with:
> make u-boot.itb

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Tested-by: Mark Kettenis <kettenis@openbsd.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/firefly-rk3399_defconfig b/configs/firefly-rk3399_defconfig
index b8d0654..ab25015 100644
--- a/configs/firefly-rk3399_defconfig
+++ b/configs/firefly-rk3399_defconfig
@@ -10,6 +10,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 CONFIG_SPL_LOAD_FIT=y
+CONFIG_SPL_FIT_GENERATOR="arch/arm/mach-rockchip/make_fit_atf.py"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x4000