arm64: dts: imx8mm-u-boot.dtsi: imx8mp-u-boot.dtsi: use atf-bl31 type

Explicitly use the atf-bl31 type for the bl31.bin atf-blob. This uses
the path from the BL31 environment variable, if defined.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>