bsps/irq: Add BSP_INTERRUPT_VECTOR_COUNT

Assert BSP_INTERRUPT_VECTOR_MAX + 1 == BSP_INTERRUPT_VECTOR_COUNT.

After building all BSPs with this patch, BSP_INTERRUPT_VECTOR_MAX can be
removed and replaced by BSP_INTERRUPT_VECTOR_COUNT.  The
BSP_INTERRUPT_VECTOR_COUNT allows a default implementation which supports no
interrupt vector at all.  Using COUNT instead of MAX may avoid some
interpretation issues, for example is the maximum value a valid vector number
or not.

Update #3269.

bsps/lm32/include/bsp/irq.h

@@ -36,6 +36,7 @@
  * @brief Maximum vector number.
  */
 #define BSP_INTERRUPT_VECTOR_MAX 31
+#define BSP_INTERRUPT_VECTOR_COUNT 32
 
 /** @} */