dt-binding: can: mcp25xxfd: document device tree bindings

Add device-tree bindings for Microcip CanFD Controller mcp2517fd

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Changelog:
  V1 -> V2: new more generic name based on feedback from microchip
            cleanup of dt custom properties removing (most) gpio functions
  V2 -> V3: added vendor-prefix for gpio-opendrain
            s/_/-/
            added gpio-controller
  V3 -> V4: resend
            added: Reviewed-by: Rob Herring <robh@kernel.org>
  V7 -> V8: added support for mcp2518fd
            added microchip,gpio0-xstandby
            removed Reviewed-by because of changes

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>