net: cpsw: Add am33xx MACID readout

This patch adds a function to get the MACIDs from the am33xx SoC
control module registers which hold unique vendor MACIDs. This is only
used if of_get_mac_address() fails to get a valid mac address.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Tested-by: Steven Rostedt <rostedt@goodmis.org>