Scottish.

As a surname, Johnstone is a variant of Johnston ["John's toun or settlement"] or a spelling variation of Johnson [John's son].  The use as a forename is almost certainly a preservation of the surname for familial reasons. 

Our correspondent [JMS] has suggested that there is an outside chance that it might also be a direct translation of the French compound name Jean-Pierre which would be translated as "John-rock" and would be transliterated as "John-stone".  Stranger things have happened.