net: ethernet: ti: cpsw: move set of common functions in cpsw_priv

As a preparatory patch to add support for a switchdev based cpsw driver,
move common functions to cpsw-priv.c so that they can be used across both
drivers.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>