net: cpsw: Fix ethernet regression for dm814x

Commit b6745f6e

 ("drivers: net: cpsw: davinci_emac: move reading mac
id to common file") started using of_machine_is_compatible for detecting
type but missed at dm8148 causing Ethernet to stop working.

Let's fix the issue by adding handling for dm814x.

Cc: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Mugunthnan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>