PCI: loongson: Add ACPI init support

Loongson PCH (LS7A chipset) will be used by both MIPS-based and LoongArch-
based Loongson processors. MIPS-based Loongson uses FDT, while LoongArch-
based Loongson uses ACPI.

Add ACPI init support for the driver in pci-loongson.c because it is
currently FDT-only.

LoongArch is a new RISC ISA, mainline support will come soon, and
documentations are here (in translation):

  https://github.com/loongson/LoongArch-Documentation

Link: https://lore.kernel.org/r/20220714124216.1489304-4-chenhuacai@loongson.cn


Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>