Add support for Ubuntu RT/Low latency kernel

See here for details.

Just a note for those trying to install the Ubuntu real time kernel yourself. I just used this on my Pi5.

• Login to Ubuntu Pro, and go "Your Subscriptions" tab
• In free personal token, copy the command "sudo pro attach xxxxxx" where xxxxx is a token
• On Pi5/Pi4, run the above command
• Then run "sudo pro enable realtime-kernel --variant=raspi"
• Reboot
• In SnakeOil/Linux Kernel tab, you should see the real time kernel and can swap back to original kernel.