arm64: dts: st: add ltdc support on stm32mp255

arm64: dts: st: add ltdc support on stm32mp255

Add the LTDC node for stm32mp255 SoC and handle its loopback clocks.

ck_ker_ltdc has the CLK_SET_RATE_PARENT flag.  While having this flag is
semantically correct, it for now leads to an improper setting of the
clock rate.  The ck_ker_ltdc parent clock is the flexgen 27, which does
not support changing rates yet.  To overcome this issue, a fixed clock
can be used for the kernel clock.

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Acked-by: Yannick Fertre <yannick.fertre@foss.st.com>
Link: https://lore.kernel.org/r/20250822-drm-misc-next-v5-10-9c825e28f733@foss.st.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>