drivers: net: phy: at803x: add interface mode support

This patch adds support for RGMII TX delay configuration on Atheros 803X,
this can be enabled in debug registers. With this patch,
PHY_INTERFACE_MODE_RGMII_TXID modes are now supported.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>