arm64: dts: bitmain: lowercase unit addresses

Unit addresses are expected to be lower case. Pointed also by W=1
builds:

Warning (simple_bus_reg): /soc/serial@5801A000: simple-bus unit address format error, expected "5801a000"

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Krzysztof Kozlowski <[e-mail]>

arch/arm64/boot/dts/bitmain/bm1880.dtsi

···
 			status = "disabled";
 		};
 
-		uart1: serial@5801A000 {
+		uart1: serial@5801a000 {
 			compatible = "snps,dw-apb-uart";
 			reg = <0x0 0x5801a000 0x0 0x2000>;
 			clocks = <&clk BM1880_CLK_UART_500M>,
···
 			status = "disabled";
 		};
 
-		uart2: serial@5801C000 {
+		uart2: serial@5801c000 {
 			compatible = "snps,dw-apb-uart";
 			reg = <0x0 0x5801c000 0x0 0x2000>;
 			clocks = <&clk BM1880_CLK_UART_500M>,
···
 			status = "disabled";
 		};
 
-		uart3: serial@5801E000 {
+		uart3: serial@5801e000 {
 			compatible = "snps,dw-apb-uart";
 			reg = <0x0 0x5801e000 0x0 0x2000>;
 			clocks = <&clk BM1880_CLK_UART_500M>,