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 <[email protected]>

Author: rettichschnidi

MAINTAINERS.yml

@@ -3546,6 +3546,7 @@ Silabs SiM3U Platforms:
     - drivers/*/Kconfig.si32
     - dts/arm/silabs/sim3u*
     - dts/bindings/*/*silabs,si32*
+    - include/zephyr/dt-bindings/pinctrl/*si32*
     - soc/silabs/silabs_sim3/
   labels:
     - "platform: Silabs SiM3U"