TGPPRx: Add AF-Signalling-Protocol AVP dissection support

The AVP is defined in 3GPP TS 29.214, Table 5.3.1: Rx specific Diameter AVPs.

Change-Id: Ib0a525f97206ff61bb0d390df81e3d6ebbef59bb
Reviewed-on: https://code.wireshark.org/review/3791
Reviewed-by: Anders Broman <a.broman58@gmail.com>