v4l2-ctl can do it. Check out v4l2-ctl --help-edid.
I think the default EDIDs included in the kernel image are for 1080p60 and 2160p30 (controllable at /sys/class/hdmirx/hdmirx/edid - values 1 and 2 are the builtins). v4l2-ctl has a 2160p60 edid included, and you can also load arbitrary EDIDs from other devices - I copy the 1440p60 one from my monitor and put it on the HDMI input.
Edit to add:
v4l2-ctl --set-edid type=hdmi-4k-600mhz,audio,ycbcr444,ycbcr422,fl-fr
Should make your HDMI input show up as 2160p60 (4k), with PCM audio input on 2 channels marked front-left and front-right.