net: spacemit: Add K1 Ethernet MAC

net: spacemit: Add K1 Ethernet MAC

The Ethernet MACs found on SpacemiT K1 appears to be a custom design
that only superficially resembles some other embedded MACs. SpacemiT
refers to them as "EMAC", so let's just call the driver "k1_emac".

Supports RGMII and RMII interfaces. Includes support for MAC hardware
statistics counters. PTP support is not implemented.

Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Tested-by: Junhui Liu <junhui.liu@pigmoral.tech>
Tested-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Link: https://patch.msgid.link/20250914-net-k1-emac-v12-2-65b31b398f44@iscas.ac.cn
Signed-off-by: Paolo Abeni <pabeni@redhat.com>