Yes I have seen this, and actually all the interfaces that are off the Core Board have pins documented in this external interface doc.
There is also some documentation of the serial commands for YC1175 (thats bundled with the linux image), however I suspect this is probably incomplete. For example this chip also controls maskROM mode that you can activate through the ewelink app. Additionally there is a Bluetooth interface on this chip (and antenna on motherboard).
There is no documentation for the RK809 however, I2C0 seems to control the PMIC part, but not clear how sound is connected, presumably there is an I2S bus for audio and I2S or PDM for the Mic.
I extracted the DTS files from the iHost Linux image which seems to just be using the reference RV1126-EVB v13 which is the Rockchip Evaluation board, they probably changed a few things, but also left a lot of stuff in there that this board doesnt have such as video etc.
I currently have the previously mentioned iHost Linux Distro installed on the eMMC. I also have the debug console wired up and am trying to get sd-card boot to work. The provided image doesnt work on sdcard, atleast if I just dd it to a card.