LoongArch: Increase MAX_IO_PICS up to 8

Begin with Loongson-3C6000, the number of PCI host can be as many as
8 for multi-chip machines, and this number should be the same for I/O
interrupt controllers. To support these machines we also increase the
MAX_IO_PICS up to 8.

Cc: stable@vger.kernel.org
Tested-by: Mingcong Bai <baimingcong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>