hsr: enhance netlink socket interface to support PRP

Parallel Redundancy Protocol (PRP) is another redundancy protocol
introduced by IEC 63439 standard. It is similar to HSR in many
aspects:-

 - Use a pair of Ethernet interfaces to created the PRP device
 - Use a 6 byte redundancy protocol part (RCT, Redundancy Check
   Trailer) similar to HSR Tag.
 - Has Link Redundancy Entity (LRE) that works with RCT to implement
   redundancy.

Key difference is that the protocol unit is a trailer instead of a
prefix as in HSR. That makes it inter-operable with tradition network
components such as bridges/switches which treat it as pad bytes,
whereas HSR nodes requires some kind of translators (Called redbox) to
talk to regular network devices. This features allows regular linux box
to be converted to a DAN-P box. DAN-P stands for Dual Attached Node - PRP
similar to DAN-H (Dual Attached Node - HSR).

Add a comment at the header/source code to explicitly state that the
driver files also handles PRP protocol as well.

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