arm64: dts: rockchip: make rk3368 grf syscons simple-mfds

The general register files do contain a lot of separate functions and
while some really are only registers with a lot of different 1-bit
settings, there are also a lot of them containing some bigger function
blocks. To be able to define these as sub-devices, make them simple-mfds.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: David Wu <david.wu@rock-chips.com>

arch/arm64/boot/dts/rockchip/rk3368.dtsi

@@ -632,7 +632,7 @@ mbox: mbox@ff6b0000 {
 	};
 
 	pmugrf: syscon@ff738000 {
-		compatible = "rockchip,rk3368-pmugrf", "syscon";
+		compatible = "rockchip,rk3368-pmugrf", "syscon", "simple-mfd";
 		reg = <0x0 0xff738000 0x0 0x1000>;
 	};
 
@@ -645,7 +645,7 @@ cru: clock-controller@ff760000 {
 	};
 
 	grf: syscon@ff770000 {
-		compatible = "rockchip,rk3368-grf", "syscon";
+		compatible = "rockchip,rk3368-grf", "syscon", "simple-mfd";
 		reg = <0x0 0xff770000 0x0 0x1000>;
 	};