i'm co-maintainer for the BCM2835 ARM architecture. I take care of upstreaming as much as possible changes for the Raspberry Pi. I'm very happy that the foundation guys now sent there changes to linux-rpi-kernel, but i also want to invent other developers and testers.
As an inspiration here is a list of the current TODOs. But every patch (improvement or bugfix) is welcome.
In upstream the rules for changes are harder, so it will be necessary that you already have some experience in kernel development.