tests: drivers: spi loopback on nucleo_l476 board

This configures the nucleo_l476rg board for testing the SPI loopback
when with DMA transfer.
It requires pin MOSI to be connected to pin MISO of the SPI1
SPI tx/rx are assigned to the request ID 1 of dma1 channels 3 and 2

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