MIPS: Provide a symbol for the legacy performance counter interrupt.

Based on https://patchwork.linux-mips.org/patch/3576 - but this really
deserves its own patchset and the symbol should also be used :)
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/irq.h

@@ -136,6 +136,7 @@ extern void free_irqno(unsigned int irq);
  * IE7.  Since R2 their number has to be read from the c0_intctl register.
  */
 #define CP0_LEGACY_COMPARE_IRQ 7
+#define CP0_LEGACY_PERFCNT_IRQ 7
 
 extern int cp0_compare_irq;
 extern int cp0_compare_irq_shift;

arch/mips/kernel/traps.c

@@ -1597,7 +1597,7 @@ void __cpuinit per_cpu_trap_init(bool is_boot_cpu)
 			cp0_perfcount_irq = -1;
 	} else {
 		cp0_compare_irq = CP0_LEGACY_COMPARE_IRQ;
-		cp0_compare_irq_shift = cp0_compare_irq;
+		cp0_compare_irq_shift = CP0_LEGACY_PERFCNT_IRQ;
 		cp0_perfcount_irq = -1;
 	}