dt-bindings: pinctrl: stm32: Introduce HDP

'HDP' stands for Hardware Debug Port, it is an hardware block in
STMicrolectronics' MPUs that let the user decide which internal SoC's
signal to observe.
It provides 8 ports and for each port there is up to 16 different
signals that can be output.
Signals are different for each MPU.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://lore.kernel.org/20250711-hdp-upstream-v7-1-faeecf7aaee1@foss.st.com


[Fixed up Clement's new email address]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>