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.com


Signed-off-by: Mark Brown <broonie@kernel.org>