dts: stm32f0: fix few warnings

Fixes following warnings:
- zephyr.dts:419.10-426.5: Warning (simple_bus_reg): /soc/clocks:
missing or empty reg/ranges property
- zephyr.dts:477.27-484.5: Warning (simple_bus_reg): /soc/serial@40011C00:
simple-bus unit address format error, expected "40011c00"

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>

dts/arm/st/f0/stm32f071.dtsi

@@ -7,6 +7,15 @@
 #include <st/f0/stm32f051.dtsi>
 
 / {
+	clocks {
+		clk_hsi48: clk-hsi48 {
+			#clock-cells = <0>;
+			compatible = "fixed-clock";
+			clock-frequency = <DT_FREQ_M(48)>;
+			status = "disabled";
+		};
+	};
+
 	soc {
 		compatible = "st,stm32f071", "st,stm32f0", "simple-bus";
 
@@ -16,15 +25,6 @@
 			};
 		};
 
-		clocks {
-			clk_hsi48: clk-hsi48 {
-				#clock-cells = <0>;
-				compatible = "fixed-clock";
-				clock-frequency = <DT_FREQ_M(48)>;
-				status = "disabled";
-			};
-		};
-
 		pinctrl: pin-controller@48000000 {
 			gpioe: gpio@48001000 {
 				compatible = "st,stm32-gpio";

dts/arm/st/f0/stm32f091.dtsi

@@ -43,9 +43,9 @@
 			status = "disabled";
 		};
 
-		usart8: serial@40011C00 {
+		usart8: serial@40011c00 {
 			compatible = "st,stm32-usart", "st,stm32-uart";
-			reg = <0x40011C00 0x400>;
+			reg = <0x40011c00 0x400>;
 			clocks = <&rcc STM32_CLOCK_BUS_APB2 0x00000080>;
 			resets = <&rctl STM32_RESET(APB2, 7U)>;
 			interrupts = <29 0>;