My best guess is that you should do the following:
1. Create a linux-imx_%.bbappend recipe in your layer
(assuming you have one).
2. Add a do_install_append function which overrides
linux-imx is the name of the kernel recipe
if you are using an NXP BSP. If you are using another
BSP the name of the kernel recipe might be different,
please adjust as required.
Prior to proceeding to this you should check
that you can see your files in your Yocto build
The content of the /boot partition normally
includes content from the kernel-devicetree package
(created by the kernel recipe), because these are
installed under /boot in the rootfs.
After making the changes suggested above
just cleanall + rebuild your kernel and verify that
the DTB file has been properly overwritten (look again
at this same directory I point you to above, compare
file checksums to ensure the copy worked).
If that worked, then just cleanall your
final image, and rebuild it.
Hopefully this will work for you.
I hope this helps.
Kind regards, Laurent.