Theoretically, you would install Linaro as per the normal instructions, and then recompile the kernel as per the instructions here: Linaro Releases (Under the “Building the Linux kernel from source” section). This will give you a new boot file that you can load using fastboot.
When recompiling the kernel, you want to first edit this file to include the uarts that you want: qrb5165-rb5.dts « qcom « dts « boot « arm64 « arch - working/qualcomm/kernel.git - Qualcomm Landing Team kernel
I would use the uart12 example in that file to guide you how to enable other uarts. After loading the new boot-rb5.img file with the new uarts, I would think it would show up in /dev/ (hopefully there aren’t any additional steps involved).
Hopefully this works - I haven’t really been able to get any peripherals to work myself (see this post: Enabling SPI on the RB5 - #6 by Juzasonos).
Good luck - I wish this forum was more helpful in providing assistance for these kinds of things.