arm64: dts: mediatek: mt7981b-openwrt-one: Remove useless cells from flash@0

arm64: dts: mediatek: mt7981b-openwrt-one: Remove useless cells from flash@0

In spi2's flash@0 there is only one `partitions` subnode: this
alone makes specifying address and size cells useless, but then
this subnode has no address and no size, which even makes the
currently declared address/size cells wrong.

Fixes: 869b3bb5ada2 ("arm64: dts: mediatek: mt7981b-openwrt-one: Enable SPI NOR")
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>