dt-bindings: atmel-gpbr: add microchip,sam9x60-gpbr

Add microchip,sam9x60-gpbr to DT bindings documentation.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-12-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -3,7 +3,9 @@
 The GPBR are a set of battery-backed registers.
 
 Required properties:
-- compatible:		"atmel,at91sam9260-gpbr", "syscon"
+- compatible:		Should be one of the following:
+			"atmel,at91sam9260-gpbr", "syscon"
+			"microchip,sam9x60-gpbr", "syscon"
 - reg:			contains offset/length value of the GPBR memory
 			region.