arm64: dts: mediatek: mt7981b: OpenWrt One: set console UART to 115200

arm64: dts: mediatek: mt7981b: OpenWrt One: set console UART to 115200

Use the new uart0 label for the console and make the speed explicit by
setting stdout-path = "serial0:115200n8" under /chosen. This keeps the
DTS OS-agnostic: no bootargs or distribution-specific properties are
added.

Drop the 'current-speed' property from uart0 as it is not allowed by the
mediatek UART binding. The baud is already provided via stdout-path.

Verification: Boot-tested with mainline Image+DTB via U-Boot on OpenWrt
One (MT7981B). Serial console active at 115200, DTB decompile
confirms serial0 alias and stdout-path set correctly.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202509211032.0rJjPoYE-lkp@intel.com/
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>