dt-bindings: phy: Add bindings for HiKey 970 PCIe PHY

Document the bindings for HiKey 970 (hi3670) PCIe PHY
interface, supported via the pcie-kirin driver.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/461495431dd28ad2779658659f137db4bd747aa8.1628061310.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>