ARM: dts: mstar: align UART node name with bindings

Bindings expect UART/serial node names to be "serial".

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

arch/arm/boot/dts/mstar-v7.dtsi

···
 				status = "disabled";
 			};
 
-			pm_uart: uart@221000 {
+			pm_uart: serial@221000 {
 				compatible = "ns16550a";
 				reg = <0x221000 0x100>;
 				reg-shift = <3>;