Well, when all else fails, resort to the O.E.D. I did that and discovered a number of things.

There is no entry for "Sally Gardens" or "Salley Gardens".

"Sally" might be a corruption of a number of different words relating to willows, acacias and gum trees. However, all the species it refers to seem to be antipodal, I think all from Australia.

There is a third meaning for "sally" deriving from the military term that gave us "sally ports" in castle walls and "sallies" out against an enemy. Sally can be used to mean a breaking out of emotion in an unaccustomed way, i.e. letting loose. It is not much of a jump from there to a place near a village that is the "Lover's Lane".

Ah, ambiguity.

Sourdough