dt-bindings: atmel-usart: Remove wildcard

Remove chip whildcard and introduce the list of compatibles instead.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/atmel-usart.txt

 * Atmel Universal Synchronous Asynchronous Receiver/Transmitter (USART)
 
 Required properties for USART:
-- compatible: Should be "atmel,<chip>-usart" or "atmel,<chip>-dbgu"
-  The compatible <chip> indicated will be the first SoC to support an
-  additional mode or an USART new feature.
-  For the dbgu UART, use "atmel,<chip>-dbgu", "atmel,<chip>-usart"
+- compatible: Should be one of the following:
+	- "atmel,at91rm9200-usart"
+	- "atmel,at91sam9260-usart"
+	- "atmel,at91rm9200-dbgu", "atmel,at91rm9200-usart"
+	- "atmel,at91sam9260-dbgu", "atmel,at91sam9260-usart"
 - reg: Should contain registers location and length
 - interrupts: Should contain interrupt
 - clock-names: tuple listing input clock names.