How hierarachical device files with respect to Yocto Project.

As an example, Poky's beaglebone.conf file, which specifies the creation of three related .dtb files:

...
KERNEL_IMAGETYPE = "zImage"
KERNEL_DEVICETREE = "am335x-bone.dtb am335x-boneblack.dtb am335x-bonegreen.dtb"
KERNEL_EXTRA_ARGS += "LOADADDR=${UBOOT_ENTRYPOINT}"
...

#include "am33xx.dtsi"
#include "am335x-bone-common.dtsi"
#include "am335x-boneblack-common.dtsi"

/ {
        model = "TI AM335x BeagleBone Black";
        compatible = "ti,am335x-bone-black", "ti,am335x-bone", "ti,am33xx";
};

&cpu0_opp_table {
        /*
         * All PG 2.0 silicon may not support 1GHz but some of the early
         * BeagleBone Blacks have PG 2.0 silicon which is guaranteed
         * to support 1GHz OPP so enable it for PG 2.0 on this board.
         */
        oppnitro-1000000000 {
                opp-supported-hw = <0x06 0x0100>;
        };
};