* Add bindings to allow creating multiple physical layouts that specify
their key's physical attributes, and the matching matrix transform
and dependant kscan to use.
* Synthesize a basic physical layout if none specified, for backwards
compatibility.
* Update matrix transform API to explicitly pass in the selected transform
to the API instead of using a fixed chosen transform.
* Move kscan subscription and handling into the physical layout code, so
that selecting a different physical layout at runtime can also use the
correct kscan instance.
* Add `physical_layouts.dtsi` file to include so you can use the
pre-configured `&key_physical_attrs` for adding you layout keys.
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