arm: am57xx: cl-som-am57x: add ETH support

Add MAC support.

Use PHY, connected to RGMII1 as a default Eth adapter,
by appropriate setting of 'cpsw_data.active_slave'.

'cpsw_phy' env variable can override this setting.

Set the MAC addresses in the U-Boot environment.
The addresses are retrieved from the on-board EEPROM or from the SOC's
MAC fuses.

Set the following PHYs RGMII clock delays:
- Enable RX delay
- Disable TX delay

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
[uri.mashiach@compulab.co.il: add RGMII clock delays]
Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>