Documentation: dt: document all the atmel pmc compatibles

Documentation for atmel-pmc only list one compatible, add the remaining
compatible strings.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

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

 * Power Management Controller (PMC)
 
 Required properties:
-- compatible: Should be "atmel,at91rm9200-pmc"
+- compatible: Should be "atmel,<chip>-pmc".
+	<chip> can be: at91rm9200, at91sam9260, at91sam9g45, at91sam9n12,
+	at91sam9x5, sama5d3
+
 - reg: Should contain PMC registers location and length
 
 Examples: