net: dwc-xlgmac: add module license

When building the driver as a module, we get a warning about the
lack of a license:

WARNING: modpost: missing MODULE_LICENSE() in drivers/net/ethernet/synopsys/dwc-xlgmac.o
see include/linux/module.h for more information

Curiously the text in the .c files only mentions GPLv2+, while the license
tag in the PCI driver contains both GPL and BSD. I picked the license text
as the more definite reference here and put a GPL tag in there.

Fixes: 65e0ace2

 ("net: dwc-xlgmac: Initial driver for DesignWare Enterprise Ethernet")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>