Enable eth driver model for dra7xx_evm as cpsw supports
driver model.

This was already added with the commit 641b936f but with
commit bd724584 to add fit support CONFIG_DM_ETH was missed.
Signed-off-by: NMugunthan V N <mugunthanvnm@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: NTom Rini <trini@konsulko.com>
Acked-by: NLokesh Vutla <lokeshvutla@ti.com>