Bluetooth: Add convenience function for fetching IRKs

There are many situations where we need to check if an LE address is an
RPA and if so try to look up the IRK for it. To simplify such cases this
patch adds a convenience function for the job.

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