Add a new port: devel/libpru.

libpru is a library to interface with Programmable Realtime Units on
FreeBSD. Currently only the Texas Instruments PRUs are
supported. Those can be found in the AM18XX and AM33XX (BeagleBone
Black) SoCs.

Differential Revision: https://reviews.freebsd.org/D1266
Reviewed by: bapt
Approved by: bapt