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>