net: ethernet: mtk_wed: add support for devices with more than 4GB of dram

Introduce WED offloading support for boards with more than 4GB of
memory.

Co-developed-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/1c7efdf5d384ea7af3c0209723e40b2ee0f956bf.1700239272.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>