arm64: dts: mediatek: mt7981b: add labels for UART0/1/2 nodes

arm64: dts: mediatek: mt7981b: add labels for UART0/1/2 nodes

Add stable labels (uart0, uart1, uart2) to the MT7981B SoC UART nodes so
board DTS files can reference them directly. This change is purely
cosmetic and introduces no functional differences.

Verification: Built dtbs and boot-tested mainline Image+DTB via U-Boot on
MT7981B hardware; decompiled DT shows the uart0 label present and the
serial0 alias (or absolute path) resolves to serial@11002000.

Signed-off-by: Bryan Hinton <bryan@bryanhinton.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>