dt-bindings: mfd: ocelot: Remove spi-max-frequency from required properties

The property spi-max-frequency was initially documented as a required
property. It is not actually required, and will break bindings validation
if other control mediums (e.g. PCIe) are used.

Remove this property from the required arguments.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221025050355.3979380-2-colin.foster@in-advantage.com