net: stmmac: dwmac-loongson: Add new multi-chan IP core support

net: stmmac: dwmac-loongson: Add new multi-chan IP core support

Add a new multi-chan IP core (0x12) support which is used in Loongson-
2K3000/Loongson-3B6000M. Compared with the 0x10 core, the new 0x12 core
reduces channel numbers from 8 to 4, but checksum is supported for all
channels.

Add a "multichan" flag to loongson_data, so that we can simply use a
"if (ld->multichan)" condition rather than the complicated condition
"if (ld->loongson_id == DWMAC_CORE_MULTICHAN_V1 || ld->loongson_id ==
DWMAC_CORE_MULTICHAN_V2)".

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Henry Chen <chenx97@aosc.io>
Tested-by: Biao Dong <dongbiao@loongson.cn>
Signed-off-by: Baoqi Zhang <zhangbaoqi@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Link: https://patch.msgid.link/20250424072209.3134762-3-chenhuacai@loongson.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>