Bluetooth: controller: Cond. compile LE Encryption

Add Kconfig option to be able to conditional compile the
Bluetooth v4.0 LE Encryption procedure.

This is needed in order to be able to not support encryption
on nRF51 series when using Data Length Update procedure with
upto 251 byte payloads until a software-based CCM support is
implemented.

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