In Westeros Knighthood is closely bound to the Faith of the Seven and as the majority of the North still follows the Old Gods they don't appear to have knights. There appears to be at least one exception to the rule with maybe a second.
Ser Bartimus: He's a follower of the Old Gods but has been knighted.
Davos could not argue with the truth of that. From what he had seen at Eastwatch-by-the-Sea, he did not care to know winter either. "What gods do you keep?" he asked the one-legged knight.
"The old ones." When Ser Bartimus grinned, he looked just like a skull. "Me and mine were here before the Manderlys. Like as not, my own forebears strung those entrails through the tree."
A Dance with Dragons, Davos IV
Ser Jorah Mormont: Though it isn't clear what Gods he follows he's of the North and a knight so is likely another exception to the rule.
George R. R. Martin has also commented on this saying Knighthood is exclusive to those who follow the Seven, though of course we have the previous exceptions.
Can someone who keeps to the old gods be made a knight too or is it a exclusive of the Seven?
The latter. Those who follow the old gods can be the northern equivilent of knights, but it's not quite the same.
Here, George states that there is a Northern equivalent to being a Knight, do we know what that is?