defconfig: am437x_sk_evm: enable eth driver model

Enable eth driver model for am437x_sk_evm as cpsw supports
driver model.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>