dts: st: n6: add stm32n6 pinctrl dtsi

Adds STM32N6 pinctrl dtsi files.

(cherry picked from commit 1d1f81866ccbaa6e84e9960ed763e005d1e45560)

Original-Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>
GitOrigin-RevId: 1d1f81866ccbaa6e84e9960ed763e005d1e45560
Cr-Build-Id: 8724818813958597281
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8724818813958597281
Copybot-Job-Name: zephyr-hal_stm32-copybot-downstream
Change-Id: I0cd734a85689d22eb2cffca6e7975c2888069e77
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/hal_stm32/+/6197773
Bot-Commit: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Commit-Queue: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>