At a high level there a few things you need to do:
- rebuild the stock device tree, and kernel then install to make sure you now how to do this.
- make modifications to the device tree to describe your new hardware
- (possibly) add an an appropriate driver to the kernel for your new device
- rebuild the modified device tree and kernel and install
Since you have built the board, I am assuming you are familiar with the hardware. The 410 processor only has one MIPI-DSI output, there is a MUX on the board to route it either to the onboard DSI-to-HDMI converter, or to the DSI pins on the high speed expansion connector. you can flip the dipswitch to force MIPI-DSI to go to the highspeed expansion connector.
Since you flashed the latest Linaro onto the board, I am going to assume you are planning to run Linaro Debian on the board. For today start with step 1) once you can recreate the stock kernel, we will help you with steps 2) and 3).
The basic build instructions are here: http://builds.96boards.org/releases/dragonboard410c/linaro/debian/latest/ we normally use a Linux box to do the build, but you can actually do the build on the DragonBoard if you don’t have a Linux box. Of course if you brick the board and have to reload it, you might loose your work.
Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.