test: drivers: spi stm32 loopback test running on the stm32f746 nucleo

Define the overlay to run the tests/drivers/spi/spi_loopback/
on the nucleo_f746zg board.
The DMA 2 is used for spi1 transfer.
Like on nucleo_f767zi, connect pin D11 to D12 (SPI1 MOSI to MISO)

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