tests: drivers: spi loopback on nucleo_l152 with DMA

Configure through an overlay file, the nucleo_l152re board
for running the tests/drivers/spi/spi_loopback with DMA.
Limit the spi speed freq to run the test correctly.
The SPI1 is selected: MOSI/MISO output pins are available on the platform.
DMA1 channel 2 & 3 are selected for SPI1.
Connect pin D11 & D12 of CN5 to PASS the test.
Add the spi_loopback fixture.

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