arm64: dts: imx8mm: use common binman configuration

With the move to using binman to generate SPL aka u-boot-spl-ddr.bin and
U-Boot proper aka u-boot.itb every board now covers such configuration
in its own U-Boot specific device tree include. Move the comon part of
that configuration to the common imx8mm-u-boot.dtsi include file.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>