Bluetooth: Add tty HCI driver

tty_hci driver exposes a /dev/hci_tty character device node, that intends to
emulate a generic /dev/ttyX device that would be used by the user-space
Bluetooth stacks to send/receive data to/from the WL combo-connectivity
chipsets.

The device driver has no internal logic of its own to intrepret data & all
such logic is handled by the user-space stack.