bluetooth: hci_uart: add LL protocol serdev driver support

Turns out that the LL protocol and the TI-ST are the same thing AFAICT.
The TI-ST adds firmware loading, GPIO control, and shared access for
NFC, FM radio, etc. For now, we're only implementing what is needed for
BT. This mirrors other drivers like BCM and Intel, but uses the new
serdev bus.

The firmware loading is greatly simplified by using existing
infrastructure to send commands. It may be a bit slower than the
original code using synchronous functions, but the real bottleneck is
likely doing firmware load at 115.2kbps.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: linux-bluetooth@vger.kernel.org
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>