I've been having this same issue with my x360 m6ar004dx. It has an AMD FX-9800p, and the exact same ELAN touch screen device. All works in Windows, but doesn't show up in Linux.
In my dmesg, I found an error related to the GPIO. After scouring for possible solutions, I stumbled on this gem the other day:
https://bugzilla.kernel.org/show_bug.cgi?id=198715#c14
I tried the patch on the latest kernel in Arch Linux, and the GPIO error is gone, and the ELAN device is recognized in xinput.
The problem now is it still doesn't work :( I tried a cat on all of the input events, and sadly I get nothing from the touchscreen. Further, the wacom settings don't recognize any devices.
So, while I feel this patch got us 2 steps closer, I'm not sure what to do next. Perhaps someone will figure it out.