spi: dt-bindings: atmel,at91rm9200-spi: Add DMA related properties

The DT nodes of the SPI IP's may contain DMA related properties so
make sure that the binding is able to properly validate those as
well by making it aware of these optional properties.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220913142205.162399-5-sergiu.moga@microchip.com