arm64: dts: mt8183: add capacity-dmips-mhz

Pinned the frequency to the max and run dhrystone to get the value.
little cpu: 11071 (max freq: 1989000)
big cpu: 15293 (max freq: 1989000)

11071 : 15293 ~= 741 : 1024

Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>