add errata to ietf-ipfix-psamp.yang

[pjaitken]

Add errata 4843 and errata 4909 to ietf-ipfix-psamp.yang

[einarnn]

@pjaitken we don't normally take PRs here for either vendor or IETF models. Since the IETF maintain their own editing process, the best path is usually via the WG for the model, followed by the automated sync. @xorrkaz may want to comment more.

[pjaitken]

@einarnn The base model was defined in RFC6728. Unfortunately IANA have not been applying errata to yang models.

[einarnn]

@pjaitken can you get the IETF to apply the errata? What I'm worried about is that if we merge your updates we will then have two sources for the model.