devicetree: fix incorrect internal usage

`@internal`/`@endinternal` is used to put "internal" information within
docstring blocks, but not to hide from public documentation any
macro/function. For this purpose, we use `@cond`/`@endcond` blocks with
the INTERNAL_HIDDEN convention.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>