net: ethernet: ti: cpsw: move cpdma resources to cpsw_common

Every net device private struct holds links to shared cpdma resources.
No need to save and every time synchronize these resources per net dev.
So, move it to common driver struct.

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>