dt-bindings: trivial-devices: Remove fsl,mc13892

This device's bindings are not trivial: Additional properties are
documented in in Documentation/devicetree/bindings/mfd/mc13xxx.txt.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>