net: ethernet: ti: cpsw: move links on h/w registers to cpsw_common

The pointers on h/w registers are common for every cpsw_private
instance, so no need to hold them for every ndev.

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>