phy: phy-hi3670-usb3: move driver from staging into phy

The phy USB3 driver for Hisilicon 970 (hi3670) is ready
for mainstream. Mode it from staging into the main driver's
phy/ directory.

Acked-by: Rob Herring <robh@kernel.org>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/dcf66a01aaeaab93cda52f9a283ecbdf9fa71bb8.1624606660.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>