need advice for installing electrical outlet inside soffit/eave
I suppose if you put an access panel in the soffit and a receptacle inside the attic. As far as the NEC is concerned there doesn't seem to be a problem. However if you install a piece of equipment with a standard cord connection on the exterior and run that connection through the soffit. That does violate the NEC. 

I may not be correct, but I imagine everyone including myself who has viewed this post is wondering, "If you are cutting in an access panel in the soffit, why not just install a duplex flush in the soffit?". 

Personally I am against installing devices in remote, concealed areas since they could potentially become faulty and create a fire hazard. There is a potential that this type of condition could go unobserved until a fire is out of hand.

Hope this helps and good luck