Bluetooth: btmtkuart: add an implementation for boot-gpios property

Not every platform has the pinctrl device integrates the GPIO the function
such as MT7621 whose pinctrl and GPIO are separate hardware so the driver
adds additional boot-gpios to let the MT766[3,8]U can enter the proper boot
mode by gpiod for such platform.

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