net: dwc-xlgmac: declaration of dual license in headers

The driver "dwc-xlgmac" is dual-licensed. This patch adds
declaration of dual license in file headers.

Signed-off-by: Jie Deng <jiedeng@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net: dwc-xlgmac: Initial driver for DesignWare Enterprise Ethernet

Synopsys provides a new DesignWare Core Enterprise Ethernet MAC
IP (DWC-XLGMAC) for Ethernet designs. It is compliant with the
IEEE 802.3-2012 specifications, including IEEE 802.3ba and
consortium specifications.

This patch provides the initial 25G/40G/50G/100G Ethernet driver
for Synopsys XLGMAC IP Prototyping Kit.

Signed-off-by: Jie Deng <jiedeng@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>