Bluetooth: hci_h5: Add ability to allocate memory for private data

In some cases uart-base drivers may need to use priv data. For
example, to store information needed for devcoredump.

Fixes: 044014ce ("Bluetooth: btrtl: Add Realtek devcoredump support")
Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>