Bluetooth: mediatek: add support for MediaTek MT7663S and MT7668S SDIO devices

This adds the support of enabling MT7663S and MT7668S SDIO-based
Bluetooth function.

There are quite many differences between MT766[3,8]S and standard
Bluetooth SDIO devices such as Type-A and Type-B devices. For example,
MT766[3,8]S have its own SDIO registers layout, definition, SDIO packet
format, and the specific flow should be programmed on them to complete
the device initialization and low power control and so on.

Currently, there are many independent programming sequences from the
transport which are exactly the same as the ones in btusb.c about MediaTek
support [1] and btmtkuart.c. We can try to split the transport independent
Bluetooth setups on the advance, place them into the common files and allow
varous transport drivers to reuse them in the future.

[1] http://lists.infradead.org/pipermail/linux-mediatek/2019-January/017074.html

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