dt-bindings: mtd: ti,am654: Prevent unevaluated properties

Reference mtd-physmap.yaml which contains all the relevant properties
for this device. Add "unevaluatedProperties: false" to avoid any
spurious addition of random properties.

Cc: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20230619092916.3028470-18-miquel.raynal@bootlin.com