```
TARGET=arm MACHINE=arm make builddtb FDT_DTS_FILE=stm32mp157c-ev1.dts
```
EDIT:
Since the import of Linux 6.5 device-tree sources, dts files are under a vendor-specific directory. Use the full path for now, so it can be found:
```
TARGET=arm MACHINE=arm make builddtb FDT_DTS_FILE=/usr/src/sys/contrib/device-tree/src/arm/st/stm32mp157c-ev1.dts
```