dt-bindings: mfd: Move embedded controllers to own directory

dt-bindings: mfd: Move embedded controllers to own directory

Move several embedded controller bindings (like ChromeOS EC, Gateworks
System Controller and Kontron sl28cpld Board Management) to new
subdirectory "embedded-controller" matching their purpose.

An embedded controller (EC) is a discrete component that contains a
microcontroller (i.e. a small CPU running a small firmware without
operating system) mounted into a larger computer system running
a fully fledged operating system that needs to utilize the embedded
controller as part of its operation.

So far the EC bindings were split between "mfd" and "platform"
directory.  MFD name comes from Linux, not hardware, and "platform" is a
bit too generic.

Rename Gateworks GSC and Huawei Gaokun filenames to match compatible, as
preferred for bindings.

Acked-by: Michael Walle <mwalle@kernel.org> # for sl28cpld
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Link: https://lore.kernel.org/r/20250825081201.9775-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>