Bluetooth: Add initial sleep support to Three-wire UART

This patch adds very basic support for the sleep related messages. The
only thing the code does right now is send a wakeup message as soon as
receiving a sleep one, essentially preventing the controller from going
to sleep.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>