tests: drivers: spi loopback running on stm32u585 disco kit

Configure the target board to PASS the test case.
The PCLK2 clock feeding the SPI1 is reduced by 2 (apb2 prescaler)
to accept the slow freq config.
The SPI1 MISO and MOSI pins are linked on the HW board
(on arduino CN13, D11 & D12)

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