Documentation: dts: Add bindings for APM X-Gene SoC ethernet driver

This patch adds documentation for APM X-Gene SoC ethernet DTS binding.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Ravi Patel <rapatel@apm.com>
Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>