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;