EDIT: It looks like a few changes to the qcom-ipq8064.dtsi file is all that is needed to get cpuidle working on the r7500v2 and the R7800.
To enable cpuidle on the R7800, see this post below to edit qcom-ipq8064.dtsi. See also the WARNING for R7800 testers in this post above - @anon50098793 has tested the concept on the R7800 and no "smoke" reported yet.
WARNING My patch "ipq806x: k419 cpuidle fix" contains edits for usb in qcom-ipq8064.dtsi specific to my edits in qcom-ipq8064-r7500v2.dts (also on my github site as "ipq806x: r7500v2 k419 extend overlay and usb fix") R7800 users using my patch may have usb issues.
It would be good to have a common "ipq806x k419 usb fix" that will work on all devices that use qcom-ipq8064.dtsi. I'll follow the community... See @Ansuel's post to get started adjusting the dtsi and dts files for usb on the R7800.
Lastly @Ansuel and/or @anon50098793 , assuming everything works out after sufficient testing, would you mind submitting the cpuidle pull request(s)? For personnel reasons related to privacy, I choose not to identify myself in public online forums; consequently, I can't submit the pull request per openwrt identity requirements (a policy I fully support BTW). I'm not bothered about attribution to me - a link to this thread is more than sufficient.