net: ethernet: mtk_wed: introduce mtk_wed_buf structure

Introduce mtk_wed_buf structure to store both virtual and physical
addresses allocated in mtk_wed_tx_buffer_alloc() routine. This is a
preliminary patch to add WED support for MT7988 SoC since it relies on a
different dma descriptor layout not storing page dma addresses.

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>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>