PCI: kirin: Allow building it as a module

There's nothing preventing this driver from being loaded as a module.
Change its config from bool to tristate.

Link: https://lore.kernel.org/r/b5e7cfe9df09b492750bd6db0f0c911eaae8c2d4.1634812676.git.mchehab+huawei@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Xiaowei Song <songxiaowei@hisilicon.com>