dts: arm: st: u0: fix msi clock

`msi-range` value 4 sets MSI clock frequency to 1 MHz and not 4 MHz as
intended.

Change value to 6 to increase clock to 4 MHz which matches comment.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>

Author: Jeppe Odgaard

dts/arm/st/u0/stm32u0.dtsi

@@ -64,7 +64,7 @@
 		clk_msi: clk-msi {
 			#clock-cells = <0>;
 			compatible = "st,stm32-msi-clock";
-			msi-range = <4>; /* 4MHz (reset value) */
+			msi-range = <6>; /* 4MHz (reset value) */
 			status = "disabled";
 		};