dt-bindings: mtd: spi-nand: Convert spi-nand description file to yaml

Let's get rid of spi-nand.txt by converting it to yaml schema. While at
converting this file, let's actually pull all the generic properties
from nand-chip.yaml which might apply to a SPI-NAND chip.
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-6-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/mtd/spi-nand.txt

-SPI NAND flash
-
-Required properties:
-- compatible: should be "spi-nand"
-- reg: should encode the chip-select line used to access the NAND chip

Documentation/devicetree/bindings/mtd/spi-nand.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mtd/spi-nand.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: SPI-NAND flash device tree bindings
+
+maintainers:
+  - Miquel Raynal <miquel.raynal@bootlin.com>
+
+allOf:
+  - $ref: "nand-chip.yaml#"
+
+properties:
+  compatible:
+    const: spi-nand
+
+  reg:
+    description: Encode the chip-select line on the SPI bus
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false