dt-bindings: mtd: microchip,mchp23k256: convert to DT schema

dt-bindings: mtd: microchip,mchp23k256: convert to DT schema

Convert Microchip 23K256 SPI SRAM MTD binding to YAML format.
Changes during conversion:
- Remove "address-cells" and "size-cells" from properties and
  required sections as there is no child node for sram.

Signed-off-by: Akhila YS <akhilayalmati@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>