phy: omap-control: update dra7 and am437 usb2 Documentation bindings

The dra7-usb2 and am437-usb2 bindings have not yet been used.
Change them to be more elegant.
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/phy/ti-phy.txt

@@ -9,9 +9,9 @@ Required properties:
                         e.g. USB2_PHY on OMAP5.
  "ti,control-phy-pipe3" - if it has DPLL and individual Rx & Tx power control
                         e.g. USB3 PHY and SATA PHY on OMAP5.
- "ti,control-phy-dra7usb2" - if it has power down register like USB2 PHY on
+ "ti,control-phy-usb2-dra7" - if it has power down register like USB2 PHY on
                         DRA7 platform.
- "ti,control-phy-am437usb2" - if it has power down register like USB2 PHY on
+ "ti,control-phy-usb2-am437" - if it has power down register like USB2 PHY on
                         AM437 platform.
  - reg : Address and length of the register set for the device. It contains
    the address of "otghs_control" for control-phy-otghs or "power" register