[nrf fromtree] boards nrf_bsim: Provide IRQ_PRIO_LOWEST

Some apps use it, so let's provide it.

Signed-off-by: Alberto Escolar Piedras <[email protected]>
(cherry picked from commit c4e6ec8)

Author: aescolar

boards/native/nrf_bsim/board_irq.h

@@ -18,6 +18,7 @@ void nrfbsim_WFE_model(void);
 void nrfbsim_SEV_model(void);
 
 #define IRQ_ZERO_LATENCY	BIT(1) /* Unused in this board*/
+#define IRQ_PRIO_LOWEST		UINT8_MAX
 
 #ifdef __cplusplus
 }