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>

Documentation/devicetree/bindings/mtd/cypress,hyperflash.txt

+Bindings for HyperFlash NOR flash chips compliant with Cypress HyperBus
+specification and supports Cypress CFI specification 1.5 command set.
+
+Required properties:
+- compatible : "cypress,hyperflash", "cfi-flash" for HyperFlash NOR chips
+- reg : Address of flash's memory map
+
+Example:
+
+	flash@0 {
+		compatible = "cypress,hyperflash", "cfi-flash";
+		reg = <0x0 0x4000000>;
+	};