dt-bindings: mfd: syscon: 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 <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220823101021.387034-1-krzysztof.kozlowski@linaro.org