[MIPS] Alchemy: Add au1500 reserved interrupt

In the conversion done in the commits

  95c4eb3ef4484ca85da5c98780d358cffd546b90
  9d360ab4

  [MIPS] Alchemy: Renumber interrupts so irq_cpu can work.

one reserved interrupt on au1500 was missed. this broke the au1000 ethernet
driver.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>