dt-bindings: spi: QuadSPI driver for Atmel SAMA5D2

Atmel SAMA5D2 QuadSPI driver was moved from mtd to spi subsystem,
this change is just moving DT-binding documentation.
Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Piotr Bugalski <bugalski.piotr@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>