。
Just having you come would be enough.
Literal
Come honorific-give-only [for-で] OK-[explanatory-のです].
下さる is the honorific form of くれる — 'give me (honorifically).' 下さるだけでいい = 'just your giving [the action of coming] is OK' — expressing that the speaker asks for nothing more than the act of visiting. A warm, polite sentiment.