spi: dt-bindings: snps,dw-apb-ssi: drop ref from reg-io-width

reg-io-width is a standard property, so no need for defining its type

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml

···
     const: spi
 
   reg-io-width:
-    $ref: /schemas/types.yaml#/definitions/uint32
     description: I/O register width (in bytes) implemented by this device
     default: 4
     enum: [ 2, 4 ]