LoongArch: Add PCI controller support

Loongson64 based systems are PC-like systems which use PCI/PCIe as its
I/O bus, This patch adds the PCI host controller support for LoongArch.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>