mips: dts: ralink: mt7621: add serial1 and serial2 nodes

Add serial1 and serial2 nodes to define the existence of
the MT7621's second and third UARTs.

Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.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>