Bluetooth: controller: Restrict enc to 1M 27 bytes PDU on nRF51

Restrict encryption support on nRF51 series SoC to Bluetooth
LE 1M PHY and max. 27 bytes PDU. If 251 bytes PDU using Data
Length Update procedure is desired, then LE Encryption
procedure will not be supported (until a software CCM is
implemented in future).

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>