dt-bindings: clk: at91: add bindings for SAM9X60 pmc

Add SAM9X60 PMC compatible string.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/at91-clock.txt

@@ -30,7 +30,8 @@ For example:
 Power Management Controller (PMC):
 
 Required properties:
-- compatible : shall be "atmel,<chip>-pmc", "syscon":
+- compatible : shall be "atmel,<chip>-pmc", "syscon" or
+	"microchip,sam9x60-pmc"
 	<chip> can be: at91rm9200, at91sam9260, at91sam9261,
 	at91sam9263, at91sam9g45, at91sam9n12, at91sam9rl, at91sam9g15,
 	at91sam9g25, at91sam9g35, at91sam9x25, at91sam9x35, at91sam9x5,