Second pass over TUSB power related code:

  - Add new field in board-specific platform data, giving a nonzero floor to
    the amount of VBUS power that peripherals could draw; pass that to the
    platform code.

  - Implement the call letting gadget drivers say how much VBUS power their
    current configuration may draw, and use that to initialize appropriately
    (pure-peripheral vs OTG modes) on USB reset and disconnect.

  - Provide TUSB-specific hook into OTG transceiver logic to support that
    VBUS call; and _only_ change the VBUS usage through that call.

  - Remove obsolete comment and bitmask declarations.

On one board I measured 36 mA (at 5.1V) coming from VBUS not the battery
using the default gadget drivers in non-OTG TUSB configuration.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>