E3-1240 v5 cpufreq-info says: "no or unknown cpufreq driver is active on this CPU"

Modern Intel processors use P-states for CPU frequency control. When Linux is using this driver, it will be visible thus:

# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
intel_pstate

The Debian 3.16 kernel is new enough and actually has the intel_pstate driver, so it should be working. My first suspect would be a BIOS setting. Unfortunately Dell doesn't seem to have documented these settings very well, so I can tell you the most likely place is under System Profile Settings in the BIOS, but I don't have one of these servers on hand so I can't say exactly which option might fix the problem.