dt-bindings: mtd: mxc-nand: Drop undocumented properties from example

With unevaluatedProperties issues fixed, 'nand-bus-width' and
'nand-ecc-mode' are flagged as undocumented. Removing them from the example
is the easiest solution to silence the warnings.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220525205947.2487165-1-robh@kernel.org