drivers: ethernet: stm32: Put DMA buffer to DTCM section

For STM32F7 MCU the actual implementation doesn't work when the
DMA buffers are placed in the SRAM.
This might be a problem with caches.
To overcome this problem, the buffer is moved to the DTCM.

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>