dt-bindings: spi_atmel: add microchip,sam9x60-spi

Add microchip,sam9x60-spi to DT bindings documentation.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1578488123-26127-13-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi_atmel.txt

 Atmel SPI device
 
 Required properties:
-- compatible : should be "atmel,at91rm9200-spi".
+- compatible : should be "atmel,at91rm9200-spi" or "microchip,sam9x60-spi".
 - reg: Address and length of the register set for the device
 - interrupts: Should contain spi interrupt
 - cs-gpios: chipselects (optional for SPI controller version >= 2 with the