It is located in the 4.1.15 kernel code under arch/arm/boot/dts/, just as written in the Wiki.
Here: https://github.com/varigit/linux-2.6-im ... plays.dtsi
Since you didn't find it, I guess you are using an older version of Android, in which our device tree files are organized a little differently.
In that case, you'll need to manually patch it. The relevant device tree files are imx6qdl-var-dart.dtsi and imx6q-var-dart.dts.