All the STM32F7 SoCs except the STM32F723 have an on-chip FS PHY, just
like the STM32F4 family. The STM32F723 only has an on-chip HS PHY and
does not have the FS PHY.
You can see that on RM0431, pages 1172 and 1173.
The idea is to disable the ULPI clock for SoCs which do not have an HS
PHY, as the FS PHY should be used instead. If USB_HS_PHYC is defined,
the SoC has an HS PHY but not FS PHY, when it is not defined, it has a
FS PHY but no HS PHY.
Thanks for the pointers. I updated my tree accordingly.