Bluetooth: hci_bcm: Add serdev support

Add basic support for Broadcom serial slave devices.
Probe the serial device, retrieve its maximum speed and
register a new hci uart device.

Tested/compatible with bcm43438 (RPi3).

Signed-off-by: Loic Poulain <loic.poulain@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>