power: add send channel msg function to map file

power: add send channel msg function to map file

Adding new wrapper function to existing private (but unused 'till now)
function with an rte_power_ prefix.

The plan is to clean up all the header files in the next release so
that only the intended public functions are in the map file and only
the relevant headers have the rte_ prefix so that only they are
included in the documentation.

Signed-off-by: David Hunt <david.hunt@intel.com>
Reviewed-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>