net: ethernet: ti: cpsw: move platform data and slaves info to cpsw_common

These data are common for net devs in dual_emac mode. No need to hold
it for every priv instance, so move them under cpsw_common.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>