Bluetooth: Implement proper low-power support for Three-wire UART

This patch adds on-demand wakeup request sending (and re-sendind) when
we are in low-power state. When the controller enters this state it will
send a sleep message after which the host is not allowed to send any
other packets until a wakeup request has been sent and the woken message
received as a response to it. The wakeup requests are re-sent
periodically until a woken message is received.

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