of: Print rather than WARN'ing when overlap check fails

__rmem_check_for_overlap() is called very early in boot, and on some
powerpc systems it's not safe to call WARN that early in boot.

If the overlap check fails the system will oops instead of printing a
warning. Furthermore because it's so early in boot the console is not up
and the user doesn't see the oops, they just get a dead system.

Fix it by printing an error instead of calling WARN.

Fixes: ae1add24 ("of: Check for overlap in reserved memory regions")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Rob Herring <robh@kernel.org>