Documentation: dt bindings: move ..usb/usb-phy.txt to ..phy/ti-phy.txt

Since now we have a separate folder for phy, move the PHY dt binding
documentation of TI to that folder.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>