examples/l3fwd-power: add baseline PMD management mode

examples/l3fwd-power: add baseline PMD management mode

The PMD Power Management scheme currently has 3 modes,
scale, monitor and pause. However, it would be nice to
have a baseline mode for easy comparison of power savings
with and without these modes.

This patch adds a 'baseline' mode were the PMD power
management is not enabled. Use --pmd-mgmt=baseline.

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>