arm: dts: mt7623: fix available memory size on bananapi-r2

There is 2GB DDR3 available on bananapi-r2 board as [1] specified.

[1] http://www.banana-pi.org/r2.html



Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>