Bluetooth: controller: Use SERIES config instead of NRF52

In preparation for the new Nordic MDK and nRF52840 IC support we switch
to using the SERIES config instead of the old "NRF52" macro that will be
deprecated in the new MDK.

JIRA: ZEP-1418

Change-Id: I563f025c9db9b7497116c5af23814d95c720f836
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>