ARM: at91/dt: sam9g20: set at91sam9g20 pllb driver

The at91sam9g20 SOC uses its own pllb implementation which is different
from the one inherited from at91sam9260 SOC.

Signed-off-by: Gaël PORTAY <gael.portay@gmail.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9g20.dtsi

@@ -40,6 +40,7 @@
 				};
 
 				pllb: pllbck {
+					compatible = "atmel,at91sam9g20-clk-pllb";
 					atmel,clk-input-range = <2000000 32000000>;
 					atmel,pll-clk-output-ranges = <30000000 100000000 0 0>;
 				};