devicetree: fix incorrect internal usage
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 <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr