net: phy: mscc: add support for VSC8502 in dual RGMII mode

The VSC8502 is a Microchip (formerly Microsemi, formerly Vitesse)
dual port, gigabit Ethernet copper PHY which supports the MII, GMII and
RGMII MAC-side interfaces.

Of these, I could only test RGMII, and my board needed RGMII delays to
be applied by software, so I am able to confirm that this patch handles
that properly.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>