dt-bindings: samsung: Document bindings for Midas family boards

Document GT-I9300, GT-I9305, GT-N7100, and GT-N7105 bindings, along
with the shared "midas" binding.

Signed-off-by: Simon Shields <simon@lineageos.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>