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/MAINTAINERS b/board/compulab/imx8mm-cl-iot-gate/MAINTAINERS
index 9c6b170..9db1fb6 100644
--- a/board/compulab/imx8mm-cl-iot-gate/MAINTAINERS
+++ b/board/compulab/imx8mm-cl-iot-gate/MAINTAINERS
@@ -4,3 +4,4 @@
 F:	board/compulab/imx8mm-cl-iot-gate/
 F:	include/configs/imx8mm-cl-iot-gate.h
 F:	configs/imx8mm-cl-iot-gate_defconfig
+F:	configs/imx8mm-cl-iot-gate-optee_defconfig