PCI: kirin: Add support for a PHY layer

PCI: kirin: Add support for a PHY layer

The pcie-kirin driver contains both PHY and generic PCI driver.

The best would be, instead, to support a PCI PHY driver, making the driver
more generic.

However, it is too late to remove the Kirin 960 PHY, as a change like that
would make the DT schema incompatible with past versions.

So, add support for an external PHY driver without removing the existing
Kirin 960 PHY from it.

Link: https://lore.kernel.org/r/f38361df2e9d0dc5a38ff942b631f7fef64cdc12.1634812676.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>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Xiaowei Song <songxiaowei@hisilicon.com>