service: return integer service id from register

service: return integer service id from register

This commit reworks the service register function to accept
an extra parameter. The parameter is a uint32_t *, which when
provided will be set to the integer service_id that the newly
registered service is represented by.

This is useful for services that wish to validate settings at
a later point in time - they need to know their own service id.

This commit updates the eventdev sw pmd, as well as unit tests
to use the new register API.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>