dt-bindings: net: bluetooth: add boot-gpios property to UART-based device

Not every platform has the pinctrl device integrates the GPIO the function
such as MT7621 whose pinctrl and GPIO are separate hardware so adding an
additional boot-gpios property for such platform allows them to bring up
the device.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>