dt-bindings: at91-sama5d2_adc: add microchip,sam9x60-adc

Add microchip,sam9x60-adc to DT bindings documentation.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-7-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/iio/adc/at91-sama5d2_adc.txt

 * AT91 SAMA5D2 Analog to Digital Converter (ADC)
 
 Required properties:
-  - compatible: Should be "atmel,sama5d2-adc".
+  - compatible: Should be "atmel,sama5d2-adc" or "microchip,sam9x60-adc".
   - reg: Should contain ADC registers location and length.
   - interrupts: Should contain the IRQ line for the ADC.
   - clocks: phandle to device clock.