How can I find the Device Tree
For many variscite-related questions the variwiki for your board is a great place to start: Variwiki DART 6UL
If you click on Release Notes you get the exact Kernel Version used in the Kirkstone Build of Yocto. The Devicetree is part of the Linux Kernel repository. They are in your case under arch/arm/boot/dts and in newer versions in arch/arm/boot/dts/nxp/imx/imx6ul*.dts
Folder with Devicetrees (keep in mind that gitlab only shows 1000 elements, so either search for the exact name or clone the repo)
How can I edit the Device Tree
There is a page how to do this in the variwiki: Customizing the Linux Kernel
Basically you have the option to replace the kernel used by yocto with your own customized kernel, or to use a patch that modifies the default yocto kernel.
How do I enable the SPI Pin for the DART-6UL board
How to add SPI to DART 6UL - there is another variwiki page for this.