I've try kernel without memset() on the board - is ok, board boot.
All drivers works fine. Than I grep the kernel sources:

[root@windmill linux]# grep -nri "memset(irq_desc" arch/
arch/mips/au1000/common/irq.c:449:      memset(irq_desc, 0, sizeof(irq_desc));
arch/mips/ite-boards/generic/irq.c:184:        memset(irq_desc, 0, sizeof(irq_desc));
[root@windmill linux]#

Only 2 matches! Does we needs memset() at all?
And if some one try to initialize irq_desc from start_kernel() before arch_init_irq() call, then following arch_init_irq() call discard all that initialization.