d/p/imx8: Fix missing ATF file in binman image generation

Binman is called passing the atf-bl31 file path with the following parameter:

atf-bl31-path=/usr/lib/arm-trusted-firmware/imx8mn/bl31.bin

However, in order to use that path, the atf_blob in u-boot dts must be defined as type = "atf-bl31";