spi: stm32f0 IRQ priority is invalid

Executing spi_loopback test for f0 based boards (stm32f072b_disco) with assertions enabled fails:

```bash
ASSERTION FAIL [prio <= ((1 << 2) - 1)] @ /home/dw_dev/0_zephyr/arch/arm/core/irq_manage.c:115:
        invalid priority 5! values must be less than 4
```

in dts/arm/st/stm32f0.dtsi interrupt priority is set to 5, but should be lower for f0 series.