bsp/arm/tms570: protect against vector < 2 in irq_set_priority

calls to tms570_irq_set_priority protect against requesting a
priority < 2, but not against inputs with vector < 2.

for example calling tms570_irq_set_priority(ESM_HIGH, 10)
will not only get through the guard, it will attempt to write
to CHANCTRL0 (which gets blocked by hardware), puts whatever
was there (apparently hardcoded INT-REQ0) into channel 10,
and returns success.

So the channel 10 request dissappeared from the VIM channel
map completely - yet this call returns RTEMS_SUCCESSFUL

TRM (for both TMS570LS31x and TMS570LC43x) states:
NOTE: CHAN0 and CHAN1 are hard wired to INT_REQ0 and
INT_REQ1, so they cannot be remapped.

This commit puts a guard to fail on requests with vector < 2.

bsps/arm/tms570/include/bsp/irq.h

@@ -152,6 +152,8 @@
  * channel associated with the priority is assigned to this channel.  The
  * specified interrupt vector is assigned to the channel associated with the
  * priority.  So, this function swaps the channels of two interrupt vectors.
+ * Interrupt vectors 0 and 1 cannot be reprioritized because the corresponding
+ * VIM channels are hardwired to INT_REQ0 and INT_REQ1 (FIQ requests).
  *
  * @param vector is the number of the interrupt vector to set the priority.
  *
@@ -160,7 +162,7 @@
  * @retval ::RTEMS_SUCCESSFUL The requested operation was successful.
  *
  * @retval ::RTEMS_INVALID_ID There was no interrupt vector associated with the
- *   number specified by ``vector``.
+ *   number specified by ``vector``, or the vector cannot be remapped.
  *
  * @retval ::RTEMS_INVALID_PRIORITY The interrupt priority specified in
  *   ``priority`` was invalid.

bsps/arm/tms570/irq/irq.c

@@ -79,6 +79,11 @@ rtems_status_code tms570_irq_set_priority(
    return RTEMS_INVALID_ID;
   }
 
+  /* CHAN0 & CHAN1 are hard wired to INT_REQ0 & INT_REQ1 and can't be remapped. */
+  if (vector < 2) {
+    return RTEMS_INVALID_ID;
+  }
+
   if (priority < 2) {
     return RTEMS_INVALID_PRIORITY;
   }