net: ethernet: mtk_eth_soc: add rxd_size to mtk_soc_data

Similar to tx counterpart, introduce rxd_size in mtk_soc_data data
structure.
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>