hwmon: (amd_energy) Restore visibility of energy counters

hwmon: (amd_energy) Restore visibility of energy counters

Commit 60268b0e8258 ("hwmon: (amd_energy) modify the visibility of
the counters") restricted visibility of AMD energy counters to work
around a side-channel attack using energy data to determine which
instructions are executed. The attack is described in 'PLATYPUS:
Software-based Power Side-Channel Attacks on x86'. It relies on quick
and accurate energy readings.

Limiting energy readings to privileged users is annoying. A much better
solution is to make energy readings unusable for attacks by randomizing
the time between updates. We can do that by caching energy values for
a short and randomized period of time.

Cc: Naveen Krishna Chatradhi <nchatrad@amd.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>