USB: gadget: pxa25x: basic transceiver support

This adds very basic otg_transceiver support, with vbus_session
and vbus_draw callbacks.

Now VBUS sensing can be handled by an external driver which registers
the otg_transceiver interface. It also allows gadget drivers to configure
the current drawn from VBUS. The UDC driver just passes their requests
along to the transceiver driver.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>