phy/marvell: add 88e1121 interface mode support

This patch adds support for RGMII RX/TX delay configuration on marvell 88e1121
and derivatives.  With this patch, PHY_INTERFACE_MODE_RGMII_*ID modes are now
supported on these devices.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>