Bluetooth: Add skeleton functions for debugfs creation

The debugfs file creation has been part of the core initialization
handling of controllers. With the introduction of Bluetooth 4.2 core
specification, the number of debugfs files is increasing even further.

To avoid cluttering the core controller handling, create a separate
file hci_debugfs.c to centralize all debugfs file creation. For now
leave the current files in the core, but in the future all debugfs
file creation will be moved.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>