MAINTAINERS: Cover Si32 pinctrl dt binding header

Without this change, `get_maintainer.py path ...` did not return a SiM3U
specific maintainer information when querying for
include/zephyr/dt-bindings/pinctrl/si32-pinctrl.h.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>