net: ethernet: mtk_eth_soc: add txd_size to mtk_soc_data

In order to remove mtk_tx_dma size dependency, introduce txd_size in
mtk_soc_data data structure. Rely on txd_size in mtk_init_fq_dma() and
mtk_dma_free() routines.
This is a preliminary patch to add mt7986 ethernet support.

Tested-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>