net: ethernet: ti: cpsw: introduce mac sl module api

The MAC SL submodule has a lot of common functions between many of TI SoCs
AM335x/AM437x/DRA7(AM57xx), Keystone 2 66AK2HK/E/L/G and K3 AM654, but
there are also differences especially in registers offsets and sets of
supported functions.

This patch introduces the MAC SL submodule API which is intended to provide
a common way to access the MAC SL submodule and hide HW integrations
details.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>