imx8mm-cl-iot-gate: Split the defconfigs

Currently imx8mm-cl-iot-gate_defconfig fails to produce a working boot
binary due to the lack of fip.bin:

"  BINMAN  all
Image 'main-section' is missing external blobs and is non-functional: blob-ext

Some images are invalid"

To make the build process more consistent with the other i.MX8M targets,
split the defconfig in two:

- imx8mm-cl-iot-gate_defconfig: standard defconfig that only
requires ATF / DDR firmware.

- imx8mm-cl-iot-gate-optee_defconfig: "more advanced" defconfig that
requires ATF / Optee / mbedtls / DDR firmware.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
diff --git a/board/compulab/imx8mm-cl-iot-gate/Kconfig b/board/compulab/imx8mm-cl-iot-gate/Kconfig
index 30760cb..e6ceb91 100644
--- a/board/compulab/imx8mm-cl-iot-gate/Kconfig
+++ b/board/compulab/imx8mm-cl-iot-gate/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_IMX8MM_CL_IOT_GATE
+if TARGET_IMX8MM_CL_IOT_GATE || TARGET_IMX8MM_CL_IOT_GATE_OPTEE
 
 config SYS_BOARD
 	default "imx8mm-cl-iot-gate"