net: ethernet: mtk_eth_soc: add support for in-SoC SRAM

net: ethernet: mtk_eth_soc: add support for in-SoC SRAM

MT7981, MT7986 and MT7988 come with in-SoC SRAM dedicated for Ethernet
DMA rings. Support using the SRAM without breaking existing device tree
bindings, ie. only new SoC starting from MT7988 will have the SRAM
declared as additional resource in device tree. For MT7981 and MT7986
an offset on top of the main I/O base is used.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/e45e0f230c63ad58869e8fe35b95a2fb8925b625.1692721443.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>