Pending official update from STM, add memory barriers to the Ethernet
HAL code for the STM32F7xx family.
Cortex-M7 has a merging write buffer that is not automatically flushed
by accesses to devices, so without these DMBs, we sometimes lose synch
with the transmitter.
The DMBs are architecturally needed in every version of this HAL, but
adding just to the STM32F7 version for now to clear test, as the
problem has only been observed on Cortex-M7-based devices.
Fixes#5622.