ARM: at91: Document armv7m compatibles

Introduce necessary compatibles to describe the armv7m based SoCs.
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/arm/atmel-at91.txt

@@ -41,6 +41,36 @@ compatible: must be one of:
        - "atmel,sama5d43"
        - "atmel,sama5d44"
 
+ * "atmel,samv7" for MCUs using a Cortex-M7, shall be extended with the specific
+   SoC family:
+    o "atmel,sams70" shall be extended with the specific MCU compatible:
+       - "atmel,sams70j19"
+       - "atmel,sams70j20"
+       - "atmel,sams70j21"
+       - "atmel,sams70n19"
+       - "atmel,sams70n20"
+       - "atmel,sams70n21"
+       - "atmel,sams70q19"
+       - "atmel,sams70q20"
+       - "atmel,sams70q21"
+    o "atmel,samv70" shall be extended with the specific MCU compatible:
+       - "atmel,samv70j19"
+       - "atmel,samv70j20"
+       - "atmel,samv70n19"
+       - "atmel,samv70n20"
+       - "atmel,samv70q19"
+       - "atmel,samv70q20"
+    o "atmel,samv71" shall be extended with the specific MCU compatible:
+       - "atmel,samv71j19"
+       - "atmel,samv71j20"
+       - "atmel,samv71j21"
+       - "atmel,samv71n19"
+       - "atmel,samv71n20"
+       - "atmel,samv71n21"
+       - "atmel,samv71q19"
+       - "atmel,samv71q20"
+       - "atmel,samv71q21"
+
 Chipid required properties:
 - compatible: Should be "atmel,sama5d2-chipid"
 - reg : Should contain registers location and length