Welcome to the forum @OllieTCG !
This sounds like an easy question, but it is complicated. Here is where Niantic explained how they established Power Spots:
If the restaurant is an import for Powerspots, then you can submit it. If it is in the Lightship database already, then it will be marked duplicate if you submit it.
If you give the location, someone who has access to the database can tell you if it is a full-fledged Wayspot or just an import.
The best thing most Pokemon Go players can use to try to decide is to first check with the toggle on in the nomination map screen. You will see ! for both full fledged Lightship Wayspots and for the imports for Powerspots. Usually Powerspot imports will not have a photo, like this:
and I was able to add that one as a Wayspot.
But if they have a photo like this:
then they most likely are already full Wayspots and will be a duplicate.
However, they have recently added the ability to add photos to Powerspots, so this may not always be the case.
And even if you can submit it without it being marked duplicate, being able to be a pokestop or gym depends on the S2 cells. Someone can also check that for you if you give the location. If you haven’t heard about how Pokemon Go uses S2 cells, this is a good article:
https://pokemongohub.net/post/article/comprehensive-guide-s2-cells-pokemon-go/