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/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index ccaf106..dfda4cb 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -138,6 +138,13 @@
 	select IMX8MM
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4
+
+config TARGET_IMX8MM_CL_IOT_GATE_OPTEE
+	bool "CompuLab iot-gate-imx8 with optee support"
+	select BINMAN
+	select IMX8MM
+	select SUPPORT_SPL
+	select IMX8M_LPDDR4
 endchoice
 
 source "board/beacon/imx8mm/Kconfig"