error occurs when there's no clock enabled for the subsystem:
devmem2 0x4006b064 w 0x0000000f
unlocks read of Ethernet registers
root@col-vf50:~# devmem2 0x400d0024
/dev/mem opened.
Memory mapped at address 0x76f93000.
Read at address 0x400D0024 (0x76f93024): 0xF0000000