dt-bindings: pinctrl: document access-controllers property for stm32 HDP

dt-bindings: pinctrl: document access-controllers property for stm32 HDP

HDP being functional depends on the debug configuration on the platform
that can be checked using the access-controllers property, document it.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Linus Walleij <linusw@kernel.org>
Link: https://lore.kernel.org/r/20260226-debug_bus-v6-2-5d794697798d@foss.st.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>