net: cpsw: Add a minimal cpsw-common module for shared code

Looks like davinci_emac and cpsw can share some code although the
device registers have a different layout.

At least the code for getting the MAC address using syscon can
be shared by passing the register offset. Let's start with that
and set up a minimal shared cpsw-shared.c.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>