dt-bindings: trivial-devices: Allow 'spi-max-frequency' property

Trivial SPI devices can also specify 'spi-max-frequency' properties, so
allow it in the trivial devices schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210316194939.3529018-1-robh@kernel.org