usb: usb: dsps: update device tree bindings

The support for both am335x-USB instances required changes to the device
tree bindings. This patch reflects these changes in the bindings
document.

v3…v4:
- remove the child node for USB. This is driver specific on won't be
  reflected in the device tree
- use the "mentor" prefix instead of "mg".
- use "dr_mode" istead of "mg,port-mode" for the port mode. The former
  is used by a few other drivers.

v2…v3:
- use proper usb-phy nodes in evm, bone and evmsk device tree.

v1…v2:
- use mg prefix for the Metor Graphics specific attributes
- use power in mA not in mA/2 as specifed in the USB2.0 specification
- use usbX-phy instead of usbX_phy
- use dma-controller instead of dma

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>