There hasn't been any developments in the Linux kernel by Marvell CPU maintainers for the Armada 38x regarding power saving features. This issue from 2015 still hasn't been solved even in 2019 (5.4-rc2).
Changing the frequencies won't help the electrical current draw if the processor does not go to a lower power state to match it. Hence:
Reading around (can't be bothered to look it up again), there is a hardware design flaw leading to crashes/stalls once the CPU goes to a lower power state. Once it goes down to a lower power/idle state, it will never go out of it.
Besides, the thing already has a beefy heatsink for most consumer routers. And frequency scaling may also have a slight adverse effect on traffic shaping (quote from one of the sqm-scripts maintainers).
For this:
Linksys WRT AC series have a Cortex a9 CPU. The changes to a53 and a73 will only really be interesting if you're running an ESPRESSObin, MACCHIATObin, or Clearfog board. The changeset affecting us is:
+CONFIG_CC_HAS_ASM_GOTO=y
-CONFIG_USB_XHCI_PCI=y
ASM_GOTO was moved to the kernel config from a preprocessor macro throughout the Linux kernel code. This was an unnoticed regression when moving from 4.14 -> 4.19. As for, USB_XHCI_PCI, it is a driver that was obsoleted by a newer one in 4.19 (formerly used solely by mamba [WRT1900ACV1] devices I think).