ARM: dts: imx6sl: fix rng node

rng DT node was added without a compatible string.

i.MX driver for RNGC (drivers/char/hw_random/imx-rngc.c) also claims
support for RNGB, and is currently used for i.MX25.

Let's use this driver also for RNGB block in i.MX6SL.

Fixes: e29fe21c ("ARM: dts: add device tree source for imx6sl SoC")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sl.dtsi

@@ -939,8 +939,10 @@ memory-controller@21b0000 {
 			};
 
 			rngb: rngb@21b4000 {
+				compatible = "fsl,imx6sl-rngb", "fsl,imx25-rngb";
 				reg = <0x021b4000 0x4000>;
 				interrupts = <0 5 IRQ_TYPE_LEVEL_HIGH>;
+				clocks = <&clks IMX6SL_CLK_DUMMY>;
 			};
 
 			weim: weim@21b8000 {