ARM: dts: rockchip: make rk3288-grf a simple-mfd

Similar to the pmu, the general register files contain a lot of different
setting bits grouped into general registers, but also some somewhat special
entities like the controls for some phy-blocks or the io-voltage control.
To be able to move these blocks under the grf node where they actually
belong, make it a simple-mfd.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm/boot/dts/rk3288.dtsi

@@ -754,7 +754,7 @@ cru: clock-controller@ff760000 {
 	};
 
 	grf: syscon@ff770000 {
-		compatible = "rockchip,rk3288-grf", "syscon";
+		compatible = "rockchip,rk3288-grf", "syscon", "simple-mfd";
 		reg = <0xff770000 0x1000>;
 	};