USB: pxa27x_udc: add vbus_draw callback

Add the vbus_draw() callback to inform the transceiver, if
it exists, how much current may be drawn. The decision is
taken on gadget driver side using the configuration chosen
by the host and its bMaxPower field.  Some systems can use
the host's VBUS supply to augment or recharge a battery.
(There's also a default of 100 mA for unconfigured devices,
or 8 mA if they're OTG devices.)

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>