net: ethernet: mtk_eth_soc: use generic allocator for SRAM

Use a dedicated "mmio-sram" node and the generic allocator
instead of open-coding SRAM allocation for DMA rings.
Keep support for legacy device trees but notify the user via a
warning to update, and let the ethernet driver create the
gen_pool in this case.

Co-developed-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/c2b9242229d06af4e468204bcf42daa1535c3a72.1751461762.git.daniel@makrotopia.org


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