mtd: devices: add devicetree documentation for microchip 48l640

The Microchip 48l640 is a 8KByte EERAM connected via SPI.
Add devicetree bindings documentation.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210607033909.1424605-2-hs@denx.de

Documentation/devicetree/bindings/mtd/microchip,mchp48l640.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/mtd/microchip,mchp48l640.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Microchip 48l640 (and similar) serial EERAM bindings
+
+maintainers:
+  - Heiko Schocher <hs@denx.de>
+
+description: |
+  The Microchip 48l640 is a 8KByte EERAM connected via SPI.
+
+  datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/20006055B.pdf
+
+properties:
+  compatible:
+    items:
+      - const: microchip,48l640
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency: true
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      eeram@0 {
+        compatible = "microchip,48l640";
+        reg = <0>;
+        spi-max-frequency = <20000000>;
+      };
+    };
+...