Bluetooth: Implement mgmt_load_irks command

This patch implements the Load IRKs command for the management
interface. The command is used to load the kernel with the initial set
of IRKs. It also sets a HCI_RPA_RESOLVING flag to indicate that we can
start requesting devices to distribute their IRK to us.

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