kernel: MIPS: perf: ath79: Fix perfcount IRQ assignment

kernel: MIPS: perf: ath79: Fix perfcount IRQ assignment

Currently it's not possible to use perf on ath79 due to genirq flags
mismatch happening on static virtual IRQ 13 which is used for
performance counters hardware IRQ 5.

On TP-Link Archer C7v5:

           CPU0
  2:          0      MIPS   2  ath9k
  4:        318      MIPS   4  19000000.eth
  7:      55034      MIPS   7  timer
  8:       1236      MISC   3  ttyS0
 12:          0      INTC   1  ehci_hcd:usb1
 13:          0  gpio-ath79   2  keys
 14:          0  gpio-ath79   5  keys
 15:         31  AR724X PCI    1  ath10k_pci

 $ perf top
 genirq: Flags mismatch irq 13. 00014c83 (mips_perf_pmu) vs. 00002003 (keys)

On TP-Link Archer C7v4:

         CPU0
  4:          0      MIPS   4  19000000.eth
  5:       7135      MIPS   5  1a000000.eth
  7:      98379      MIPS   7  timer
  8:         30      MISC   3  ttyS0
 12:      90028      INTC   0  ath9k
 13:       5520      INTC   1  ehci_hcd:usb1
 14:       4623      INTC   2  ehci_hcd:usb2
 15:      32844  AR724X PCI    1  ath10k_pci
 16:          0  gpio-ath79  16  keys
 23:          0  gpio-ath79  23  keys

 $ perf top
 genirq: Flags mismatch irq 13. 00014c80 (mips_perf_pmu) vs. 00000080 (ehci_hcd:usb1)

This problem is happening, because currently statically assigned virtual
IRQ 13 for performance counters is not claimed during the initialization
of MIPS PMU during the bootup, so the IRQ subsystem doesn't know, that
this interrupt isn't available for further use.

So this patch fixes the issue by simply booking hardware IRQ 5 for MIPS PMU.

Signed-off-by: Petr Štetiar <ynezz@true.cz>