Bluetooth: Add delayed init sequence support for UART controllers

This patch makes it possible to have UART drivers perform an internal
initialization before calling hci_register_dev. This allows moving a lot
of init code from user space (hciattach) to the kernel side, thereby
creating a more controlled/robust initialization process.

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