Bluetooth: controller: Handle remote version as a prio event

The remote version information event needs to be processed by the
prio_recv_thread() thread in order to unblock the Host RX thread
(effectively hci_driver's recv_thread()) when it blocks waiting for a
response to a remote version information.

Add the same time gate the inclusion of the feature behind a new Kconfig
option: CONFIG_BT_REMOTE_INFO.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>