ARM: dts: armada-375: Fixup pinctrl DT warnings

pinctrl has a ranges property, so the unit name should contain an
address.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/boot/dts/armada-375.dtsi

@@ -307,7 +307,7 @@ uart1: serial@12100 {
 				status = "disabled";
 			};
 
-			pinctrl: pinctrl {
+			pinctrl: pinctrl@18000 {
 				compatible = "marvell,mv88f6720-pinctrl";
 				reg = <0x18000 0x24>;