net: ethernet: ti: cpsw: create common struct to hold shared driver data

This patch simply create holder for common data and as a start moves
pdev var to it.

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>