mips: dts: ralink: mt7621: reorder serial0 properties

Reorder serial0 properties according to the guidelines laid
out in Documentation/devicetree/bindings/dts-coding-style.rst

Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>