intr: Add Interrupt Manager implementation group

The shared BSP interrupt controller support code actually implements
parts of the Interrupt Manager.

Update #3706.

bsps/mips/shared/irq/irq.c

@@ -1,7 +1,7 @@
 /**
  * @file
  *
- * @ingroup bsp_interrupt
+ * @ingroup RTEMSImplClassicIntr
  *
  * @brief Interrupt support.
  */