Bluetooth: mediatek: add support for MediaTek MT7663U and MT7668U UART devices

This adds the support of enabling MT7663U and MT7668U Bluetooth function
running on the top of btmtkuart driver.

There are a few differences between MT766[3,8]U and MT7622 where
MT766[3,8]U are standalone devices based on UART transport while MT7622
bluetooth is a built-in device on MediaTek SoC communicating with the host
through BTIF serial transport. Thus, extra setup sequence is necessary
for these standalone devices such as remote regulator and reset control via
GPIO, baud rate changing handshake between the host and device and so on.
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>