mtd: add DT bindings for the Cortina Systems Gemini Flash

This adds device tree bindings for the Cortina systems Gemini
flash controller, a simple physmap which however need a few
syscon bits to be poked to operate properly.

Cc: Janos Laube <[e-mail]>
Cc: Paulius Zaleckas <[e-mail]>
Cc: Hans Ulli Kroll <[e-mail]>
Cc: Florian Fainelli <[e-mail]>
Cc: [e-mail]
Signed-off-by: Linus Walleij <[e-mail]>
Reviewed-by: Marek Vasut <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

Documentation/devicetree/bindings/mtd/cortina,gemini-flash.txt

···
+Flash device on Cortina Systems Gemini SoC
+
+This flash is regular CFI compatible (Intel or AMD extended) flash chips with
+some special bits that can be controlled by the machine's system controller.
+
+Required properties:
+- compatible : must be "cortina,gemini-flash", "cfi-flash";
+- reg : memory address for the flash chip
+- syscon : must be a phandle to the system controller
+- bank-width : width in bytes of flash interface, should be <2>
+
+For the rest of the properties, see mtd-physmap.txt.
+
+The device tree may optionally contain sub-nodes describing partitions of the
+address space. See partition.txt for more detail.
+
+Example:
+
+flash@30000000 {
+	compatible = "cortina,gemini-flash", "cfi-flash";
+	reg = <0x30000000 0x01000000>;
+	syscon = <&syscon>;
+	bank-width = <2>;
+};