dt-bindings: arm: samsung: document S3C6410-based boards binding

Add bindings for the FriendlyARM Mini6410 and Samsung SMDK6410 boards.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201117201106.128813-2-krzk@kernel.org