Bluetooth: btusb: Add out-of-band wakeup support

Some onboard BT chips (e.g. Marvell 8997) contain a wakeup pin that
can be connected to a gpio on the CPU side, and can be used to wakeup
the host out-of-band. This can be useful in situations where the
in-band wakeup is not possible or not preferable (e.g. the in-band
wakeup may require the USB host controller to remain active, and
hence consuming more system power during system sleep).

The oob gpio interrupt to be used for wakeup on the CPU side, is
read from the device tree node, (using standard interrupt descriptors).
A devcie tree binding document is also added for the driver. The
compatible string is in compliance with
Documentation/devicetree/bindings/usb/usb-device.txt

Signed-off-by: Rajat Jain <rajatja@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>