MIPS: Loongson 3: Add HT-linked PCI support

Loongson family machines use Hyper-Transport bus for inter-core
connection and device connection. The PCI bus is a subordinate
linked at HT1.

With LEFI firmware interface, We don't need fixup for PCI irq routing
(except providing a VBIOS of the integrated GPU).

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Hongliang Tao <taohl@lemote.com>
Signed-off-by: Hua Yan <yanh@lemote.com>
Tested-by: Alex Smith <alex.smith@imgtec.com>
Reviewed-by: Alex Smith <alex.smith@imgtec.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/6633


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>