gprs: Add encoder/decoder for the Subscriber Update Protocol (WIP)

Status: compiles and test passes

TODO:
  - Add commit message

Ticket: OW#????
Sponsored-by: On-Waves ehf
gprs: GSUP Integration (WIP)

TODO:
  - testing
  - commit msg / title
  - VTY command to supply messages in hex (testing)

Ticket: OW#????
Sponsored-by: On-Waves ehf
sgsn/doc: Add IPA default parameters

This commit adds the default protocol identifiers that should be used
for GSUP by the IPA protocol.

Sponsored-by: On-Waves ehf