Documentation: atmel-quadspi: add binding file for Atmel QSPI driver

This patch documents the DT bindings for the driver of the Atmel QSPI
controller embedded inside sama5d2x SoCs.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>