The patch does work exactly as advertised.
When I used dtc to convert CONFIG_DTB_FILE from dtb to dts, I could see that it didn't in
fact have a timer clock-frequency node. After re-creating the dtb and rebuilding Xen,
"ls /proc/device-tree/timer/" shows a clock-frequency file. When I then fire up
DomU and do the same command, it too has a clock-frequency node.