devicetree: Use `IS_EQ()` for `DT_SAME_NODE()`

`DT_SAME_NODE()` is judged by `==` and cannot be used as
a condition for `COND_CODE_1()`.
Using `IS_EQ()` to make it possible for compile-time
calculation.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>