scripts: genpinctrl: use <zephyr/...> include prefix

Zephyr has prefixed all of its includes with <zephyr/...>. While the old
mode can still be used (CONFIG_LEGACY_INCLUDE_PATH) and is still enabled
by default, it's better to be prepared for its removal in the future.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>