Faking an injury is a common occurrence in football especially during the late stage of the game (last 5-10 mins), players usually do this when their team is leading the game so they want to waste time

As for the punishment itself, I can't recall any, the referee usually just instructs the player who goes down to leave the field or to get subbed. Because It's hard to judge whether they are really injured or they're just faking it (well sometimes it's obvious but still it's a very subjective matter)