powerpc/shared: ISA bus bridge fails to enable the openpic irq

- The call to enable the openpic irq for the ISA bridge fails
  because the IRQ used is offset by the ISA bus signals and
  the openpic call expects an IRQ relative to its signals.

- Add the MVME 2600/2700 to the list of boards with an ISA bridge.

Closes #4231

bsps/powerpc/shared/irq/irq_init.c

@@ -310,7 +310,7 @@ void BSP_rtems_irq_mng_init(unsigned cpuId)
 #endif
     known_cpi_isa_bridge = 1;
   }
-  if ( currentBoard == MVME_2300 ) {
+  if ( currentBoard == MVME_2300 || currentBoard == MVME_2600_2700_W_MVME761 ) {
     /* nothing to do for W83C553 bridge */
     known_cpi_isa_bridge = 1;
   }

bsps/powerpc/shared/irq/openpic_i8259_irq.c

@@ -214,7 +214,7 @@ int BSP_setup_the_pic(rtems_irq_global_settings* config)
    	/*
      * Must enable PCI/ISA bridge IRQ
      */
-   	openpic_enable_irq (BSP_PCI_ISA_BRIDGE_IRQ);
+    openpic_enable_irq (BSP_PCI_ISA_BRIDGE_IRQ - BSP_PCI_IRQ_LOWEST_OFFSET);
 #endif
 #endif