dt-bindings: mtd: Add binding documentation for HyperFlash

Add DT binding documentation for HyperFlash devices.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>