PCI: kirin: Add power_off support for Kirin 960 PHY

In order to prepare for module unload, add a power_off method for HiKey
960.

Link: https://lore.kernel.org/r/b095818b0d7fadae4cae200f481caf7a66e61fb4.1634812676.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>