dt-bindings: arm: stm32: add required #clock-cells property

dt-bindings: arm: stm32: add required #clock-cells property

On STM32MP25 SoC, the syscfg peripheral provides a clock to the display
subsystem through a multiplexer.  Since it only provides a single clock,
the cell value is 0.

Doing so allows the clock consumers to reach the peripheral and gate the
clock accordingly.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Yannick Fertre <yannick.fertre@foss.st.com>
Acked-by: Christophe Roullier <christophe.roullier@foss.st.com>
Link: https://lore.kernel.org/r/20250822-drm-misc-next-v5-6-9c825e28f733@foss.st.com
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>