tests: drivers: spi loopback testing on the nucleo_h745zi_q boards

Connect D12(PA6) & D22(PB5) pins on the board to pass the test.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Reviewed-by: Vasilij Strassheim <vasilij.strassheim@linutronix.de>