arm/arm64: dts: st: Drop unused .dtsi

These .dtsi files are not included anywhere in the tree and can't be
tested.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20260113201340.36950-1-robh@kernel.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

Author: robherring