tests: drivers: spi loopback running on stm32f7x nucleo

This commit remove the use of the Dcache for stm32f7x
boards when running the tests on SPI with DMA.
Add the -- -DOVERLAY_CONFIG=overlay-stm32-spi-dma.conf
to run the test and connect SPI MOSI to MISO on the hw.

Signed-off-by: Francois Ramu <francois.ramu@st.com>