of: Drop superfluous dependance for OF_OVERLAY

The whole menu already depends on OF, so there is no need to additionaly specify it.

Suggested-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Rob Herring <robh@kernel.org>