ARM: at91/dt: use syscon for PMC

The PMC is not only used to drive the clocks but also has some registers
related to other functions. One of those is for example the USB gadget
bias.
Using a syscon allows to properly separate those functions.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

arch/arm/boot/dts/at91sam9260.dtsi

@@ -100,7 +100,7 @@
 			};
 
 			pmc: pmc@fffffc00 {
-				compatible = "atmel,at91sam9260-pmc";
+				compatible = "atmel,at91sam9260-pmc", "syscon";
 				reg = <0xfffffc00 0x100>;
 				interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;
 				interrupt-controller;