ARM: dts: samsung: Drop s3c6400.dtsi

This .dtsi file is not included anywhere in the tree and can't be
tested.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251212203226.458694-7-robh@kernel.org
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20260117182257.14799-2-krzk@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Author: robherring