For those how are interested, Nicolas Saenz Julienne and i working on Mainline support for the Raspberry Pi 4. The plan was to introduce initial Raspberry Pi 4 support (only debug UART, no fancy stuff like GENET, V3D, USB 3.0 or arm64) for Linux 5.4. Unfortunately there were a lot of issues (especially DMA stuff) which delayed everything to the next release. At least the pinctrl and the mmc stuff has been merged and the clock will follow soon.
Here is the issue to keep track on this: