examples/l3fwd-power: select default mode automatically

examples/l3fwd-power: select default mode automatically

Currently, the application does support running without the power
library being initialized, but it has to be specifically requested. On
platforms without support for frequency scaling using the power library,
we can just enable interrupt-only mode by default.

Suggested-by: Jerin Jacob <jerinj@marvell.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Harman Kalra <hkalra@marvell.com>