MIPS: Loongson64: Increase NR_IRQS to 320

Modernized Loongson64 uses a hierarchical organization for interrupt
controllers (INTCs), all INTC nodes (not only leaf nodes) need some IRQ
numbers. This means 280 (i.e., NR_IRQS_LEGACY + NR_MIPS_CPU_IRQS + 256)
is not enough to represent all interrupts, so let's increase NR_IRQS to
320 (NR_IRQS_LEGACY + NR_MIPS_CPU_IRQS + NR_MAX_CHAINED_IRQS + 256).

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