MIPS: Loongson64: Adjust IRQ layout

Adjust IRQ layout in order to use IRQ resources more efficiently, which
is done by adjusting NR_IRQS and MIPS_CPU_IRQ_BASE.

Before this patch:
0~15:    ISA/LPC IRQs;
16~55:   Dynamic IRQs;
56~63:   MIPS CPU IRQs;
64~127:  PCH IRQs;
128~255: Dynamic IRQs.

After this patch:
0~15:    ISA/LPC IRQs;
16~23:   MIPS CPU IRQs;
24~87:   PCH IRQs;
88~280:  Dynamic IRQs.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>