MIPS: SGI-IP27: micro-optimize arch_init_irq()

The function sets adjasted groups of bits in hub_irq_map by using
for-loops. There's a bitmap_set() function dedicated to do this.

Because [0, CPU_CALL_B_IRQ] and [NI_BRDCAST_ERR_A, MSC_PANIC_INTR]
ranges belong to the same machine word, bitmap_set() would boil down
to an inline wrapper in both cases, avoiding generating a loop, whth
the associate overhead.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>