dt-bindings: net: Add support for SpacemiT K1

The Ethernet MACs on SpacemiT K1 appears to be a custom design. SpacemiT
refers to them as "EMAC", so let's just call them "spacemit,k1-emac".

Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250914-net-k1-emac-v12-1-65b31b398f44@iscas.ac.cn


Signed-off-by: Paolo Abeni <pabeni@redhat.com>