dt-bindings: arm: fsl: Move DCFG and SCFG bindings to their own docs

In preparation to convert board-level bindings to json-schema, move
various misc SoC bindings out to their own file.

Acked-by: Shawn Guo <shawnguo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>