ARM: dts: zynq: describe SLCR as simple-mfd rather than simple-bus

Commit 9f56917a ("dm: core: make simple-bus compatible to
simple-mfd") made possible to import the following commit:

Linux commit: bc5ba9b98435bf76d92e0954da1784695aa449f1

The SLCR (System-Level Control Registers) block is an MFD (Multi
Function Device) rather than a bus.

"simple-mfd" seems a more suitable compatible string than "simple-bus".
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynq-7000.dtsi

@@ -251,7 +251,7 @@
 		slcr: slcr@f8000000 {
 			#address-cells = <1>;
 			#size-cells = <1>;
-			compatible = "xlnx,zynq-slcr", "syscon", "simple-bus";
+			compatible = "xlnx,zynq-slcr", "syscon", "simple-mfd";
 			reg = <0xF8000000 0x1000>;
 			ranges;
 			clkc: clkc@100 {