mips: dts: ralink: mt7621: associate uart1_pins with serial0

Add missing pinctrl-name and pinctrl-0 properties to declare
that the uart1_pins group is associated with serial0.

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>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>