Bluetooth: controller: Require nRF52 for privacy

Privacy on nRF51 is not passing the conformance and qualification tests
due to the time it takes to execute the privacy code while in ISR. Until
we come up with a way of optimizing and/or deferring the work, do not
allow privacy on nRF51 targets.

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