drivers: net: xgene: Add support RGMII TX/RX delay configuration

Add RGMII TX/RX delay configuration support. RGMII standard requires 2ns
delay to help the RGMII bridge receiver to sample data correctly. If the
default value does not provide proper centering of the data sample, the
TX/RX delay parameters can be used to adjust accordingly.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>